JetBlue referred to the C.D.C.’s definition of a face covering as a cloth that should fit snugly against the side of a person’s face, is secured with ties or ear loops, and includes “multiple layers of fabric for unrestricted breathing.” The C.D.C. recommends that surgical masks and N95 respirators be reserved for health care workers and other medical professionals.

The company said it would remind passengers of the new policy before their flight by email and through announcements and signage at airports. Small children who are not able to wear a face covering are exempt from the requirement, the airline said.

Sara Nelson, the president of the Association of Flight Attendants, applauded JetBlue’s new policy.

“Good job, JetBlue, for being the first U.S. airline to take this responsible step, which the C.D.C. advises will help slow the spread of the virus,” she said in a statement.

Her union has been pushing for the Department of Transportation, in coordination with the Department of Health and Human Services, to require masks for airline crews and passengers, and it has been urging people to fly as little as possible during the pandemic.

“Most U.S. airlines are now requiring flight attendants to wear a mask while working,” she said, “but we need passengers to wear masks in order to best protect everyone onboard, including crew.”