Updated: April 19, 2022 11:14 AM EDT | Originally published: February 28, 2022 1:12 PM EST

For much of the pandemic, a mask has been as essential at the airport as a boarding pass: you can’t fly without one. The same went for trains, ferries, subways, buses, and other types of public transportation.

But on April 18, a federal judge in Florida struck down the U.S. Centers for Disease Control and Prevention’s (CDC) public transportation mask mandate, arguing that it is beyond the scope of the agency’s power. The CDC had recently extended the national transportation mask mandate through May 3.

The Biden Administration says it is assessing next steps, but the court decision means that the CDC’s public transportation mask mandate is, for now, officially over.

Major U.S. airlines, including United, American, and Delta, announced that they will no longer require masks on domestic flights. Some airline officials are relieved about the decision; airline executives have for months questioned the need for in-flight mask mandates, arguing that the air filtration systems on planes are effective enough to eliminate virtually all airborne pathogens.

But even without a mandate, the CDC still recommends that people choose to wear masks on planes and other forms of public transportation. On Twitter, many public-health experts also said they will continue to wear masks on planes and other types of public transit, mandate or not.

Should you still wear a mask on planes? Here’s what the research says.

How dangerous is flying during COVID-19?

Throughout the pandemic, there have been documented instances of SARS-CoV-2 (the virus that causes COVID-19) spreading on planes.

Still, in-flight transmission happens less frequently than one might expect with such close quarters and shared air. The authors of a research review published in September 2021, which analyzed 18 prior studies on in-flight spread of SARS-CoV-2, concluded that “transmission of SARS-CoV-2 can occur in aircrafts but is a relatively rare event.”

A 2020 study conducted for the U.S. Department of Defense—and carried out by researchers from Boeing and United Airlines, among others—found that aircraft ventilation and filtration systems reduced the risk of airborne SARS-CoV-2 exposure by more than 99%. But the study didn’t account for other modes of transmission, such as direct exposure to larger respiratory droplets expelled by an infected person sitting nearby. It also did not consider the effects of walking up and down the aisles or interacting with other passengers.

It’s hard to say exactly how risky it is to fly during the pandemic, because lots of variables affect whether SARS-CoV-2 jumps from one traveler to another: everything from how much virus a traveler is carrying to the length of the flight and the vaccination statuses of other people on board. Research has shown that risk even changes depending on whether an infectious person is talking—and therefore expelling more respiratory particles—or just breathing.

The flight may not even be the riskiest part of a trip, adds Dr. Aisha Khatib, who is chair of the International Society of Travel Medicine’s Responsible Travel interest group and has studied SARS-CoV-2 transmission on flights. (The International Society of Travel Medicine’s members include travel industry employees, as well as researchers and medical experts.) There’s plenty of potential for exposure in the airport, such as when people remove their masks to eat or drink at the gate, she says. That alone makes wearing a mask a good idea while traveling.

Should I wear a mask on a plane?

You might want to continue masking up on planes, even though the CDC’s mandate is no longer in place. Research has shown that wearing a mask in public indoor spaces reduces your risk of later testing positive for COVID-19.

On planes, the air is frequently filtered, which helps keep the risk of in-flight transmission fairly low, Khatib says. But that risk drops even lower, she says, when good ventilation is accompanied by precautions including masking, symptom screening (like pre-flight temperature checks), and social distancing.

In a paper published in March 2021, researchers developed a model for predicting the infection risk aboard a plane. The most extreme scenario they considered was a 12-hour flight in which passengers were not wearing masks and one infected person was seated in tightly packed economy class. Under these conditions, the authors estimated that other economy passengers had an average infection risk of up to approximately 11%—but for someone seated in very close proximity to the infected person, the risk could rise to 99.6%.

If everyone onboard wore a surgical mask, the average risk of infection dropped as low as 3%, the authors added. Importantly, this scenario did not take into account the benefits of vaccination, which likely reduces the transmission risk further—but the findings do suggest that masks can add an extra layer of protection on flights.

Another modeling study, published in December 2021, tried to find the best airplane seating arrangement to reduce viral transmission, again by using simulations of airborne spread. If someone on a flight has COVID-19, you obviously don’t want to sit next to them, but you also don’t want to sit behind them, the researchers concluded.

“You have to look at not only east-west transmission, but also north-south transmission,” says co-author Sheldon Jacobson, a computer scientist at the University of Illinois at Urbana-Champaign who studies public health and aviation security and has received funding from the U.S. Air Force Office of Scientific Research. Keeping middle seats open and leaving some rows empty was the safest seating arrangement, his team concluded. (Airlines, however, are no longer selling flights at reduced capacities.)

Regardless of seating arrangement, masking reduced the chances of transmission onboard by about 30%, according to his team’s model.

Of course, theoretical calculations don’t always hold up in practice. In the September 2021 research review that looked at real-world studies of viral spread on planes, the authors concluded, “It is not clear whether the use of masks can prevent transmission of SARS-CoV-2 in flights.” That doesn’t mean masks do or do not work—only that published studies haven’t fully answered the question. It’s also worth noting that all of the studies in that review were published before the emergence of the highly contagious Omicron variant.

Still, some real-world data suggest that masks work on planes. “If you look at the studies, most of the in-flight transmission…occurred in flights earlier than March 2020, prior to the enforcement of in-flight mask-wearing policy,” Khatib says.

Does wearing a mask on a plane help if no one else wears one?

Khatib emphasizes that no one should travel while symptomatic, and recommends that passengers “wear a protective mask that fits well and you’ll wear consistently.” Highly protective masks, such as N95s and KN95s, can filter out almost all particles when worn correctly, so they can help keep the wearer healthy even if those around them are unmasked.

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Write to Jamie Ducharme at [email protected].

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