Delta announced the mask-wearing requirement in April.

airport guest wearing a face mask

Delta has added a total of 460 people to its no-fly list for refusing to wear masks.

Delta CEO Ed Bastian said in an email to employees that the company requires customers and employees to wear masks to reduce the transmission of the new coronavirus. Company spokesperson Morgan Durrant said Delta announced the mask-wearing requirement in April.

Bastian said in September the company banned 350 passengers that month, 240 in late August, and 100 on July 22 for refusing to follow mask protocols.

Mask enforcement is one of several Delta COVID-19 safety policies. Other protocols include blocking off seats in terminals and airplanes for social distancing, using electrostatic sprayers before flights, and handing customers sanitizing wipes.

Delta is one of two major U.S. airlines that will block middle seats on planes for the rest of 2020. Alaskan Airlines announced Thursday it extended its policy to block middle seats through Jan. 6, 2021.

Epidemiologists have told told Business Insider's Tom Pallini sitting too close together on airplanes increases the risk of contracting COVID-19 from a nearby passenger.

As coronavirus cases in the U.S. continue to climb, many companies have struggled to get customers to comply with mask protocols. Employees from several major U.S. brands reported harassment and violence directed at them when they asked shoppers to wear a mask. Retailers like Foot Locker and Dollar Tree stopped enforcing mandatory mask wearing to protect workers.

This story originally appeared on Business Insider.