Air travel has been one of the last holdouts for strict pandemic mask requirements. In the United States, for example, the mask mandate — which was recently extended to April 18, when it comes up for review again — is still enforced. Over the past year, 922 of those who didn’t wear masks received fines from the Transportation Security Administration, according to a report by the Government Accountability Office.

But there are hints that the tide may be turning: Within the past few weeks, Danish airports and London’s Heathrow Airport have lifted their mask requirements, as have several major British airlines.

Some airline employees in England rejoiced at their reclaimed freedom from enforcing mask rules at 30,000 feet. “First flight done without a mask and it was an absolute dream,” a woman, who identified herself as a flight attendant from Yorkshire, England, on her social media accounts, recently wrote on Twitter, alongside a photo of her fully visible smile.

In the United States, the International Air Transport Association, which represents nearly 300 airlines, and the U.S. Travel Association, an industry group, have been lobbying the White House not to extend the mandate further, saying it’s difficult to rationalize mask rules in the sky, given that authorities have already lifted them in other indoor locations. Republican lawmakers, who recently sued the Centers for Disease Control and Prevention to end the mask mandate for air travel, call the rule “arbitrary.” But some travel health experts and passengers say airplane cabins and airports should take a more careful approach.

“It was very unsettling,” said Rebecca Kift, 37, a clinical biochemist from Leeds, England, who had no idea that British airline TUI Airways Ltd. had lifted its mask requirement until she boarded her flight to Manchester from Spain’s Gran Canaria island recently. Because her mother is being treated for cancer, she has spent months avoiding crowded indoor situations. But there she was with four hours ahead of her in a cabin full of unmasked flight attendants and mostly unmasked passengers. “I don’t think it’s fair,” she said.

Here is a look at the confusing state of mask wear in the sky.

What just happened in England?

Unlike the United States, England never instituted a governmental mask mandate for air travel. Nonetheless, most British airlines and airports began requiring masks in June 2020, when Britain started mandating masks on other forms of transport.

Over the past couple of weeks, as parts of Britain have lifted other types of travel and mask requirements, some airports and airlines have suspended their mask rules, among them, London’s Heathrow Airport, British Airways and Virgin Atlantic. Both airlines said wearing a mask was a “personal choice,” and clarified that the shift only applies while flying to or from destinations where there are no mask requirements, such as England and Barbados.

They are not the first airlines to permit bare faces. Two additional British airlines, Jet2 and TUI Airway, had previously dropped their mask requirements, and passengers began flying without face coverings throughout Scandinavia last October.

What determines if you have to wear a mask on a particular route?

If the departure and destination countries have different restrictions, the country with the stricter rule sets the policy in the sky. Individuals flying between England and Northern Ireland on TUI Airways, for example, would not have to wear a mask, but individuals flying between England and the United States on that same airline would have to wear one.

Beyond England, Northern Ireland, Norway and Barbados, destinations that do not currently have mask requirements in the air include Mexico, St. Lucia, the Bahamas and Jamaica. The United States, Scotland, Italy and China are among the many countries that continue to require masks on planes.

Airport rules may be stricter than plane rules on a given route, meaning a traveler might have to put on a mask upon arrival. Airports in Norway, Denmark and England have been outliers in lifting mask requirements, according to the Airports Council International, a trade organization representing nearly 2,000 airports.

When can I stop flying with a mask in the United States?

Maybe on April 19, if — and that’s a big if — the White House does not extend the federal mask mandate further. Even flight attendants are divided on whether the mandate should be extended again, with some saying that the mask rule is not worth it, given the problems enforcing it, while others argue that the rule is critical to keeping medically vulnerable travelers safe.

Should COVID-19 case counts affect mask mandates?

It depends on whom you ask.

Across the world, case count numbers vary. In recent weeks, case numbers have dipped to their lowest level since June in the United States. Canada is also down to its lowest number of cases since December. But in many other places, cases are rising. As England moved away from airport and airline mask mandates on March 16, case counts hit their highest point in nearly a month. Meanwhile, a highly transmissible subvariant known as BA.2 is pummeling some parts of Asia and Europe. And on Sunday, Dr. Anthony Fauci, the Biden administration’s top adviser on the pandemic, predicted that the United States would eventually see a similar “uptick” of BA.2 cases.

“COVID is not over,” said Leonard Marcus, director of Harvard’s Aviation Public Health Initiative, which produced a report that some airline executives have used to rationalize ending mask requirements. Marcus said that it’s too soon for him to have an opinion on whether BA.2 merits an extension of the federal mask mandate. But once lifted, he said, a mask mandate will be difficult to reinstate.

Some proponents of ending mask mandates argue that so long as vaccines are preventing serious disease, case counts are irrelevant, because most international travelers are vaccinated.

But proponents of mask mandates also point out that not everyone on a plane can count on vaccines protecting them and the people they live with from severe COVID-19 infections.

Don’t advanced air filtration systems on airplanes provide enough protection?

The advanced filtration systems on many planes refresh the air every two to three minutes. Therefore the risk of being infected should be lower than in other packed indoor settings, many of which no longer require masks, airlines have argued. (It should also be lower than in the airport or on crowded bridges to a plane, where you cannot count on great ventilation systems, according to Dr. David Freedman, president-elect of the American Society of Tropical Medicine & Hygiene.)

Travel testing requirements have also made an already low-risk environment for transmission safer.

But if you’re sitting close to a contagious person, you could still end up breathing in recently emitted virus before it makes it into the air filtration system, some researchers have pointed out. “There is some evidence that passengers within two rows of an index case are at higher risk,” said Patricia Schlagenhauf, a professor of travel medicine at the University of Zurich.

Is masking on airplanes really effective?

Yes, travel experts say. While the consensus among researchers who focus on this area has been that air travel is quite safe, there are examples of coronavirus transmission on planes; most occurred before mandatory mask policies arrived, said Dr. Aisha Khatib, the chair of a group focused on responsible travel for the International Society of Travel Medicine.

“It may be argued that the most effective mitigation measure to date has been mandatory masking in-flight,” Khatib said.

Freedman echoed this point. Once the mandate goes, he said, “I’m still going to wear my N95 the whole flight.”

Some airlines and airports that have lifted mask mandates have also emphasized masks’ value. Emma Gilthorpe, Heathrow’s chief operating officer, said that even without the requirement she still would “recommend wearing them.”

Should I wear one if I’m sniffling or tested positive?

The CDC continues to advise people to quarantine for five days if they test positive, regardless of symptoms. But, inevitably, some people who test positive will still get on a plane. To reduce the risk of transmission, they should wear a good mask.

In East Asia, long before the pandemic, wearing a mask in public if a person had a cold or flu was standard etiquette, noted Emma Teng, a professor of Asian Civilization at Massachusetts Institute of Technology, who had studied the history of masks.

Teng believes that the pandemic could have a lasting effect on how Americans approach masks. “I think it’s been relatively normalized for people to wear masks in public settings if they are feeling under the weather,” she said.

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