JetBlue Airways on Monday became the first U.S. airline to announce that all passengers will have to wear a face covering on flights.

Starting May 4, passengers will be required to wear a mask that covers their nose and mouth during the duration of each flight and also during check-in, boarding and deplaning, according to a JetBlue statement.

"We are also asking our customers to follow these CDC guidelines in the airport as well," said Joanna Geraghty, the airline's president and chief operating officer. “Wearing a face covering isn’t about protecting yourself; it’s about protecting those around you.”

In early April, the Centers for Disease Control and Prevention advised people to wear face coverings "in public settings where other social distancing measures are difficult to maintain."

JetBlue customers will be reminded by email and at the airport to wear a mask. Small children who can't keep a mask on are exempt.

The airline has already started requiring flight crew members to wear face coverings on the job.

Since late March, JetBlue has also reduced the number of seats available on flights so that passengers and crew are able to spread out.

American Airlines will begin requiring flight attendants to wear masks starting May 1, the airline said in a statement Monday. Passengers will be offered personal protective equipment. Masks became mandatory for United Airlines flight attendants on Friday.

On Thursday, the Association of Flight Attendants, which represents 50,000 flight attendants across 20 airlines, sent a letter to U.S. Transportation Secretary Elaine Chao and Health and Human Services Secretary Alex Azar, asking that the departments use their "authority to mandate masks in aviation for crew, employees and passengers; require personal protective equipment; and end all leisure travel until the virus is contained."

"Since the initial outbreak of COVID-19 in China, flight attendants have been on the front lines of the growing global pandemic," the labor union's letter said. At AFA-affiliated airlines, at least 250 flight attendants have tested positive for the coronavirus, the letter said.

In a statement sent to NBC News, the Federal Aviation Administration, which operates under the Transportation Department said: "The FAA is not a public health agency, but we are lending our aviation safety expertise to federal public health agencies as they issue health guidance for crew members, including health monitoring, screening protocols and aircraft cleaning."

A CDC spokesperson said the agency is aware of the letter from the AFA. "CDC recommends that everyone wear a cloth face covering over their nose and mouth when in a community setting, including during travel if they must travel," said a statement from the CDC.

"This is an additional public health measure people should take to reduce the spread of COVID-19 in addition to (not instead of) social distancing, frequent hand washing and other everyday preventive actions."