After flight attendants and pilots criticized them for not doing more to protect employees, large airlines in the United States and around the world announced this week that they would require their crews and passengers to wear masks.

American Airlines, Delta Air Lines and United Airlines all said today that they would start requiring all passengers to wear a face covering in the coming weeks, a policy that will apply to their flight attendants and some other workers, too. The three join Lufthansa Group — which owns its namesake airline, Swiss International Air Lines and Austrian Airlines — as well as JetBlue and Frontier Airlines, all of which made similar announcements this week. Southwest Airlines has said a policy for flight attendants is coming.

As some states begin to relax or lift stay-at-home orders, lawmakers and unions representing flight attendants and pilots have stepped up calls for industrywide rules on masks to protect flight crews from passengers — and passengers from one another.

Airlines have been slow to require masks in part because they’ve been hard to come by. Early in the pandemic, many companies promised to make masks available for employees who wanted them, but some pilots and flight attendants complained that they were not always available.

“Things have changed rapidly in just two or three weeks,” Robert Isom, president of American Airlines, said in a call with journalists today. “As we take a look forward with sanitizer, we see ample supplies. With masks, it is a logistical issue of getting inventories to the right place, but we have sufficient quantities for our team.”

It will be several weeks before the airline can distribute enough sanitizer and masks to offer them to all passengers who want them, Isom said. American’s policy does not take effect until May 11.

On Wednesday, chairman of the House Transportation and Infrastructure Committee, Rep. Peter A. DeFazio, asked the head of the Federal Aviation Administration, Stephen Dickson, to require masks and take other steps to protect travelers and crews. It is “essential” that passengers wear masks and airlines space out seats, said DeFazio, D-Ore.

Two Democratic senators, Edward J. Markey of Massachusetts and Richard Blumenthal of Connecticut, urged the Transportation Department and the Department of Health and Human Services to put similar rules in place.

“The need for an emergency rule requiring face masks for all individuals engaged in air travel is both clear and urgent,” the senators wrote in a letter.

The push for a federal mask requirement has been largely driven by labor unions. In a letter last week, the Association of Flight Attendants union, which represents workers at 20 airlines, including United and Frontier, asked the transportation and health departments for an emergency regulation on masks for passengers.

Over the weekend, the union’s president, Sara Nelson, posted a photograph on Twitter, which was widely shared, showing a packed American Airlines plane with some passengers who were not wearing masks.

“ENOUGH! This was TODAY on a four hour flight. This is not okay,” Nelson wrote.

So far, the Trump administration has resisted a mask mandate.

In a statement, the FAA said that Dickson had shared DeFazio’s concerns and that the agency expected airlines to follow guidance from the Centers for Disease Control and Prevention. The Department of Health and Human Services referred a request for comment to the CDC, which repeated its general recommendations.

Studies have shown that people who don’t display symptoms of the virus can infect others. That’s why the CDC recommends that masks or face coverings be worn in “community settings,” such as grocery stores, pharmacies or the confines of an airplane.

“If you’re trying to control spread of a disease in a community, your most effective means is to control it at the source,” said Dr. Henry Wu, director of the Emory TravelWell Center in Atlanta, which provides health services for international travelers. “For an infectious disease, it would be the ill individual, primarily the ill individual who doesn’t realize they are ill.”

Since the start of the pandemic, airlines have regularly pointed to the quality of their air filtration systems, which filter and recirculate the air in the cabin every few minutes. Such systems are “quite good,” Wu said, but they can’t eliminate contagion.

“If you are sitting within 6 feet of others, obviously there are some immediate-proximity issues that even filtration systems can’t address,” he said.

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