Like many other airline passengers flying during the pandemic, Kevin Garvey thought that masks were mandatory on planes.

So when two women seated in front of him on a recent flight from Fort Myers, Fla., to Chicago dropped their face coverings, he assumed that a flight attendant would quickly remedy the situation.

It didn't happen. A crew member didn't even seem to notice that they were unmasked, he says.

"At no time did they wear their masks while talking to the flight attendant. Nor did she remind them to put on their masks," says Garvey, a retired lawyer from Chicago. "The flight attendants did nothing to enforce their own directives."

When it comes to air travel, mask rules are complicated. While airlines require face coverings, at least on paper, the rules aren’t uniformly enforced — if enforced at all. What’s more, the legal basis for requiring a mask is debatable. There are also numerous exceptions to the policies. As airlines start filling their middle seats again, these issues are deepening passenger concerns about the safety of air travel this summer.

Recently, Airlines for America (A4A), the trade group for the major U.S. airlines, announced voluntary health-related policies. The policies included an industrywide requirement that every passenger bring a face covering and wear it at the airport, on the jet bridge, and onboard the aircraft. Passengers who fail to comply may be grounded, although A4A said it will leave it to each carrier to resolve the matter according to its own policies.

"We want passengers to know that they should expect to see this added layer of protection the next time they check in for a flight," A4A chief executive Nicholas Calio said.

No federal law requires airline passengers to wear masks. Instead, airlines set their own rules, say experts. An airline can post warning signs and deny a passenger boarding or impose other penalties. As a legal basis for enforcing mask requirements, airlines cite Federal Aviation Administration regulations that say "no person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated."

But the law has its limits.

"The crew can only invoke their authority when an unruly passenger presents a threat to the safe operation of the flight," says David Gitman, president of Monarch Air Group, a private jet charter company based in Fort Lauderdale, Fla.

So are masks required by law? Strictly speaking, no.

"There is no statute, so the FAA technically can't enforce wearing masks," says Petro Kostiv, a Los Angeles lawyer and pilot. "However, airlines that require masks would more than likely win any lawsuit, because coronavirus, and how hard it has hit aviation, was not something the FAA was able to predict."

The rules, such as they are, don't apply to all passengers. For example, Delta Air Lines' list of exceptions includes:

* Passengers with a cognitive or physical disability that renders them unable to wear a mask safely.

* Travelers for whom a mask would interfere with hearing aids or implants.

* People who use self-contained oxygen.

* Children under the age of 8 who can't keep a mask on by themselves.

What's more, because of health privacy laws, Delta doesn't require passengers to disclose their medical conditions.

Delta's policy, outlined May 1 in an internal memo to its crew members, also recognizes that there may be challenges to enforcing its mask policy.

"It will not be possible for everyone to wear a face covering at all times," the airline notes. "You are not expected to police these specific situations on board, but rather to diffuse and de-escalate to the best of your ability, relying on your hospitality skills and training to achieve a positive outcome."

Delta tightened its policy on June 22, saying customers are not allowed to board an aircraft without wearing a mask. However, the current exceptions remain in place.

"Wearing a mask or face covering is one of the most important ways customers and employees stay safe while flying," says Delta spokesperson Adrian Gee. "That's why we're doubling down on our efforts to ensure customers are aware of, acknowledge, and comply with the requirement to wear a mask during boarding and throughout their flight."

Now that many airlines have stopped blocking the middle seats to help passengers maintain social distancing, conflicts are inevitable. In mid-June, American Airlines banned a passenger who refused to wear a mask. A few days later, Frontier Airlines removed a passenger from a flight for the same reason.

Expect more bannings. At least that's what you might conclude if you were a passenger on Judy Williams' recent flight from Billings, Mont., to Seattle.

"Masks were required," says Williams, a lawyer who lives in Billings. "But as soon as people were seated, I'd estimate at least 50% took them off or pulled them down."

That's also what I experienced when I flew from Los Angeles to Seattle a few weeks ago. Masks were mandatory, but on the three-hour flight, many passengers peeled them off after the seat belt light went dark. Shortly before landing, as the flight attendants came through the cabin, they hastily put their masks back on.

This policy-based and ultimately unenforceable approach to preventing the spread of covid-19 is minimally effective, industry observers say. But fixing it isn't up to the airlines.

"The real problem is the lack of federal government mandates and guidelines for minimum standards for airlines and airports to implement," says Eduardo Angeles, a former FAA associate administrator for airports who now works for the Los Angeles law firm Clark Hill. He says a piecemeal approach to mask requirements such as the ones now in place won't work.

"But, more importantly, it will not regain the confidence of passengers to return to flying," he adds.

Until then, you may not have to wear a mask on a plane. But you should.

Christopher Elliott is a consumer advocate, journalist, and co-founder of the advocacy group Travelers United. Contact him at chris@elliott.org.