The mask mandate ended midflight, sparking cheers, selfies and alarm.

Brooke Tansley heard the pilot apologize through the public address system on her flight from Atlanta to Los Angeles.

She recalled him saying, "You're going to be very excited to hear this news." Passengers were free to take off their masks if they wanted, as the Transportation Security Administration no longer required masks on planes. About a third of the people on the plane immediately removed their masks after the announcement, Ms. Tansley said.

The announcement from the T.S.A. that it would stop enforcing a mask mandate came as travelers were already on their way.

Many passengers welcomed the news with applause and cheers, as seen in videos on social media. Most fellow passengers were in wide, maskless grins when one took a celebratory selfie. The pilot told those on the flight that he had a good day.

The mask requirement was not eliminated on some flights. A flight attendant told a person that masks had to stay on.

Ms. Tansley felt a sense of fear and alarm. She had two children, one too young for a vaccine and the other too young for a mask. She and her family had undergone P.C.R. tests because they were worried about potentially infecting her colleague.

She said that she was scared and only had hope that it would be ok.

Ms. Tansley said her family hadn't been on a flight since Christmas because of the virus. She has asthma, and she wasn't sure if she would go ahead with her work meetings, or what her family would do about their return flight home on April 25.

It's not that the mask mandate has changed that upset me, it's that we boarded the plane under one set of rules, and made a decision as a family and as a work group.

Scott Hechinger, a lawyer, was waiting for his delayed flight to New York at a crowded terminal in West Palm Beach, Fla., when the announcement came that the mandate had been lifted and passengers were free to board their flights without masks.

He said that the terminal erupted in a loud, sustained cheer, and about half the travelers took off their masks.

Mr. Hechinger wrote in a text message that the announcement made him feel uncomfortable. He said that 75 percent of the time, he was unmasked.

He was worried about his wife and son who were on the separate flight.

He wrote that his wife and son would likely get this announcement midflight and be more exposed than usual.