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Early this week, Delta Air Lines made news after a plane headed to Atlanta circled back to its gate in Detroit, delaying takeoff. The crew was returning to expel two passengers who had been unwilling to follow a new but quintessential coronavirus rule.

They had refused to don masks.

That transgression is the latest addition to a bevy of infractions that can get you booted from an aircraft — even before contagion racked our world. Those no-nos vary wildly in severity and how often they're enforced, but the theoretical gamut is wide: from a joke about, say, hijacking, to smoking a cigarette ... all the way to more serious acts like transporting illegal contraband like guns or drugs.

And, then there's the 2018 case when a traveler was banned from a United Airlines flight for trying to take an "emotional support peacock" along with her. (She opted to drive across the country instead, a BBC report says.)

When it comes to the new rules for the novel coronavirus, airlines like Delta are taking them very seriously. So far, the carrier has banned 100 anti-maskers from taking their flights and gone a step further by adding them to a "no fly" list.

Delta says its strict policies about masking are part of an effort to promote best public health practices and safety amid the pandemic.

In a statement provided to NPR, Delta wrote: "Medical research tells us that wearing a mask is one of the most effective ways to reduce the COVID-19 infection rate." The airline "remains committed to requiring customers and employees to wear a mask or face covering as a consistent layer of protection across all Delta touchpoints."

And it's not just Delta. All major U.S. airlines now require passengers to wear face coverings — a dramatic change to plane etiquette. Children under age 2 and slightly older children who cannot maintain a face covering are exempt from the requirement on Delta and other airlines. Adults are generally permitted to remove a mask only when eating or drinking, though policy varies.

Many private businesses have similar rules: Be Masked or Be Gone.

Though the scientific consensus is clear and strong that masks are critical in stemming the spread of the virus, some consumers feel aggrieved by what they consider an attack on personal freedom. But according to aviation, health and legal experts, such outrage ignores a few fundamentals: In entering into agreements (read: contracts) with airline carriers (by purchasing a ticket), you're required to adhere to their policies. And that pretty much ends the matter.

In other words, Delta's no-fly list is perfectly within its scope of rights, experts stress.

The legal reasoning is pretty straightforward, says Sharona Hoffman, co-director of Case Western Reserve University's Law-Medicine Center. She puts it simply: "They're a private business, and private businesses can have rules."

So why do some people think airline mask mandates are a violation of their freedom? Part of the misconception, explains Eduardo Angeles, a lawyer who served as Federal Aviation Administration associate administrator for airports during President Barack Obama's administration, is the confusion about what exactly a "right" is.

"No one has a right to fly," Angeles says. Instead, you're just a participant navigating the free market: "You have several options [to get to your destination] — car, train, foot. And in this way, an airline is just like a restaurant: It can deny service to somebody for reasons that are specific to [it]," he says.

For those who land on the no-fly list because of mask infractions, processes to get your name removed would likely vary from airline to airline, guesses Angeles. "They have to go through their due process and appeal with the airline."

There are tons of precedent for airlines (and airports) to unilaterally enforce regulations in the name of safety, Angeles says. Just take seat belts or TSA security protocols. Airports and planes are packed with rules and limitations — even though the average person might just be so used to them that they comply without question.

Plus, for airlines, the risk posed by a passenger without a mask could be a concern, says , an attorney and physician who studies the intersection of law and medicine. She says one could theoretically argue that airlines might be bound to enforce mask-wearing as a matter of reasonable safety expectations — and failure to enforce such reasonable precautions could put them at risk of a potential suit. (Though no such case has made its way to a court.)

"In the hypothetical situation, you get sick on a flight" — and you would have to successfully prove you contracted COVID-19 because someone next to you wasn't wearing a mask — "there is an argument there that airlines have a special duty to offer passengers and crew members a degree of reasonable care with regard to the risks that would arise during the plane [ride]," she theorizes. "As a general counsel for airlines, I'd think about this."

Of course, there's a lot of legal ifs and complexities that would go into such a lawsuit — and Cantor emphasizes that she doesn't know whether it would even be successful if it went down. But the quick version of why such an argument would fly (no pun intended) has to do with the special relationship airplanes have with passengers. So the question would be: If airlines choose to ignore the medical community and make masks optional, are they arguably breaching the standard of reasonable care?

Airlines also have leeway to tinker with rules and regulations according to their health and safety priorities (as long as they aren't breaking federal anti-discrimination laws.) For example, Delta airlines has banned any mask with an exhaust valve or vent, and United and JetBlue have as well.

That's a good thing, according to Amy Price, a mask researcher and senior scientist at Stanford University's Anesthesia Informatics and Media Laboratory: Valve masks are generally a bad idea for a pretty intuitive reason, she says.

"The exhaust valve blows out concentrated virus particles through that exhaust, in essence blowing all the virus out – so it's a danger to everyone," she explains. "It's the same as wearing nothing at all [in terms of source control].

Price thinks the reason people like exhaust masks is because the ease of breathing facilitated by the valve makes it feel as if you're not wearing a mask at all ... which by the by should raise some red flags. "It gives the illusion of safety," she says.

As for exceptions to the mask rule, Delta (and many other airlines) have instituted procedures to evaluate passengers for whom masks may be medically infeasible. Delta's no-mask clearance process is "rigorous," according to the airline. It involves a "virtual consultation process facilitated by a Delta agent with a third-party medical professional [and] could take up to an hour." The airline generally encourages customers with underlying conditions that preclude mask-wearing to "reconsider travel altogether."

Meanwhile, in order to persuade people to fly at a time when air traffic has plummeted, some airlines are going a step further in creating new pandemic policies. This week, Emirates airline rolled out a policy to offer financial assistance for COVID-19 health expenses for those who test positive for the virus during their trip.

And if, God forbid, things go awry in your recovery, the airline will cover your funeral costs up to roughly $1,760.

Pranav Baskar is a freelance journalist and U.S. national born in Mumbai.

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