U.S. airlines could ban travelers from flying in the future if they refuse to wear masks on planes, an industry group said Monday.

Airlines for America, a trade organization, said a group of major American airlines will begin “vigorously” enforcing face-covering policies after reports of travelers not being held to the safety standard. Last month, several airlines acknowledged that they had told crew members to avoid escalating any confrontations in the air over mask violations and described various levels of enforcement.

Delta, Southwest, United Airlines, American Airlines, JetBlue and others will “clearly articulate” their face-covering policy to passengers and may require customers to acknowledge the policy at check-in, the association announced Monday on behalf of the member companies. If passengers don’t comply, carriers can implement their own consequences, which could include suspension of flying privileges.

“U.S. airlines are very serious about requiring face coverings on their flights,” Nicholas E. Calio, the group’s president and chief executive, wrote in the statement. “Carriers are stepping up enforcement of face coverings and implementing substantial consequences for those who do not comply with the rules."

Crew members will announce specific details once onboard, including consequences for violating the policy.

The use of face masks at airports has been sporadic, according to reports from across the country.

A tweet in May by American Airlines customer Tony Scott was shared more than 6,700 times after he wrote about a passenger seated beside him who refused to wear a mask. He said he informed a flight attendant, but no measures were taken. The airline’s website states that “a face covering is required while flying on American, except for very young children or anyone with a condition that prevents them from wearing one.”

United Airlines announced Monday that as of Thursday, any passenger who does not comply will be placed on an internal travel restriction list that will take away their travel privilege on the airline for a period of time to be determined.

A flight attendant would first tell a passenger whose face isn’t covered that it is mandatory, then offer a mask if needed. At further resistance, the employee would give the traveler a card reminding them of the in-flight mask policy. Finally, the flight attendant would file a report, kicking off a formal review. A security team would investigate and hand down a decision on future flights. The airline said flight attendants would still try to de-escalate any situation.

“Every reputable health institution says wearing a mask is one of the most effective things people can do to protect others from contracting covid-19, especially in places like an aircraft where social distancing is a challenge,” Toby Enqvist, United’s chief customer officer, said in a statement. “We have been requiring our customers to wear masks onboard United aircraft since May 4 and we have been pleased that the overwhelming majority of passengers readily comply with our policy. Today’s announcement is an unmistakable signal that we’re prepared to take serious steps, if necessary, to protect our customers and crew.”

Last month, some airlines said they would go so far as to deny boarding to passengers who refused to wear masks if they did not have any medical exemption. They emphasized the need to avoid escalating tensions in the air and said flight attendants would instead try to move passengers to other seats if they refused to cover their faces. One carrier, JetBlue, said passengers who refused to comply could be reviewed for future travel.

At the time, American Airlines got pushback for a memo it sent to pilots saying the policy would “become more lenient” once passengers were on board.

“The flight attendant’s role is informational, not enforcement, with respect to the face covering policy," the memo said. "The flight attendants are instructed not to escalate the issue if the passenger refuses to wear a face covering and to consider options, such as reseating if other passengers are involved, to defuse the situation.”

Unions representing pilots and flight attendants have called for the federal government to mandate health and safety measures such as wearing masks to give airlines more authority to enforce their policies.

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