Travelers who wish to catch a Delta flight without a mask must now undergo a medical screening before they'll be allowed on the plane, the airline said.
The step adds a layer of security for those concerned about flying as the coronavirus pandemic surges.
Passengers who can't wear masks due to health conditions "are strongly encouraged to reconsider travel," Delta said on its website. Otherwise, they'll be required to complete a virtual examination that can take an hour or longer.
"If you require this exemption, please arrive early to complete the process during check-in and avoid missing your flight," Delta said.
The airline also warned travelers against making "false claims of a disability" to get around the mask requirement, noting that travelers caught fibbing to avoid wearing masks will be barred from flying on Delta for as long as masks are required.
The screenings will be conducted over the phone by the medical consultation company STAT-MD, reported WREG, a CBS affiliate in Memphis, Tennessee.
Delta also said that passengers are welcome to wear plastic face shields in addition to masks, but that shields "are not approved mask replacements."
Most major U.S. airlines require masks to be worn on board airplanes, but enforcement has been spotty. In May, Reuters revealed that the top three carriers — Delta, American Airlines and United Airlines — told staff they could not force travelers to wear masks aboard the plane.
But the three have stepped up their rhetoric recently. American said in June that travelers refusing to wear a mask could be denied future travel.
United has said that maskless customers would "lose their travel privileges" with the airline for an undetermined amount of time. On Monday the airline announced it would increase air flow on planes when passengers board and disembark, as another measure to decrease the risk of coronavirus transmission.