Updated on Friday at 7:36 p.m. ET

Most of the nation's airlines are beginning to require passengers to wear face coverings or masks on flights to help prevent the spread of the coronavirus.

Southwest and Alaska Airlines announced Friday they will join American, Delta, United, Frontier and JetBlue in taking the action amid growing pressure from Congress and their own employees.

Flight attendants having been calling on their airlines and the federal government to require face coverings or masks on passengers since the pandemic began, and in recent weeks, some Democratic lawmakers have been urging the same, ramping up pressure on the Trump administration and the airlines to do more to protect travelers and employees from COVID-19.

The Association of Flight Attendants, which represents employees of United, Alaska, Frontier and 17 other airlines, says at least 300 of its members have contracted COVID-19 and several flight attendants have died. The union continues to call on the Federal Aviation Administration and the Department of Health and Human Services to mandate face coverings be worn by all travelers on flights and in airports.

The FAA has said it expects airlines to follow CDC recommendations that everyone wear face coverings when in public, but the regulatory agency has stopped short of requiring them.

Some airlines already require crew members to wear masks, and in addition to now requiring passengers to do the same, some airlines are blocking out middle seats, spacing out passengers during boarding and taking other steps to ensure social distancing on flights. They've stepped up their cleaning and disinfecting of airplane cabins, as well.

JetBlue was the first U.S. airline to require passengers to wear face masks, starting next week. "Wearing a face covering isn't about protecting yourself, it's about protecting those around you," said JetBlue President Joanna Geraghty. "This is the new flying etiquette."

United and several other airlines will provide masks to those passengers who do not have them.

Asked why his airline didn't act sooner when the flight attendants union had been calling for such step for over a month, United Airlines spokesman Josh Earnest said it was a supply chain issue.

"We did not want to be in a position in which we are diverting medical supplies that were badly needed for medical professionals," Earnest said. "Now that there is more inventory, we have been able to work through our supply chain to ensure that we can provide our customers with masks on May 4," when the new requirement takes effect.

Earnest would not say if the airline will deny boarding to passengers who refuse to wear a mask.

"We will be providing masks to all of our customers" to make it easy for them to comply, he said. "We do have language in our contract of carriage that will be updated, so we're going to be real clear about what the requirements are."

Earnest added that social norms are changing to the new coronavirus realities, "and I think those changing norms will carry over into the social environment on board an aircraft."

Delta will make masks mandatory on Monday, too. Frontier's requirement goes into effect May 8, and American will require passengers to wear masks as of May 11.

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