Delta has banned more than “100 people” from flying for refusing to wear a mask, according to the airline’s CEO.

Ed Bastian said that passengers who refused to comply with the carrier’s requirement that they wear face coverings at all times are being temporarily blacklisted.

“If you insist on not wearing your mask, we will insist that you don’t fly Delta,“ he told The Today Show last month.

Masks have been mandatory on Delta flights since 1 May.

The only people who are exempt are those with a relevant medical condition, something that must be established prior to the flight, during a pre-boarding process involving a consultation with a medical profession that can take up to an hour.

The airline warns passengers that ”any false claims of a disability or health condition to obtain an exemption from wearing a mask or face covering may result in the suspension of travel privileges on any Delta flight“.

The airline has temporarily stopped selling middle seats to better enable social distancing on board its flights.

As part of its stricter rules, Delta has also banned certain types of masks.

It recently updated its guidance to prohibit masks with a valve.

“Any mask with an exhaust valve is not approved as an acceptable face mask for customers travelling on any Delta operated flight,” says the policy.

This is because masks with a valve are designed to protect the wearer, not other people, as they filter what is being breathed in rather than out. They’re most commonly used in construction work or during wildfire season.

As early as May, the San Francisco Department of Public Health highlighted the issue, tweeting that masks “with the valves or openings on the front are NOT safe, and may actually propel your germs further.”

It referenced the city’s public health order, which stated: “Valves of that type permit droplet release from the mask, putting others at risk.”