What is wrong with people? Delta Air Lines now has a blacklist of over 550 customers who have refused to wear masks on flights. That’s an increase of nearly 100 in the past two weeks, and the number has almost doubled since early September. The airline has required passengers to wear masks since May but non-compliance by some ‘no-maskers’ have caused incidents and flight delays.

Customers are required to wear masks at check-in, in Sky Club lounges, and at boarding gates.

United, American and Southwest Airlines have a similar policy requiring everyone, except small children, to wear masks when not eating throughout a flight.

Customers placed on the no-fly list are banned from flying for a period determined by individual airlines.

“Fortunately, the number represents a tiny fraction of our overall customers, the vast majority of whom follow our guidelines and appreciate the steps we are taking to keep them safe and healthy,” CEO Ed Bastian wrote in an update to employees.

“Keep in mind that while developing a vaccine is an important step, widespread distribution will take many months, so we continue to expect 2021 will be a year with continued challenges. It is more important than ever to stay vigilant and work to combat the spread of the virus.”

Delta has ruled that all passengers who claim they cannot wear a face mask due to health issues must undergo a screening by telephone with a medical professional before being allowed to fly.