Several airlines are banning cloth masks, insisting that travelers wear surgical masks, FFP2 masks, or KN95 or N95 masks without a valve.

Finnish airline Finnair is the most recent to change its mask policy, saying fabric masks will not be permitted. Several other carriers, mostly those that service Europe, are giving cloth the boot, too. German carrier Lufthansa hasn’t allowed them on flights to and from Germany since Feb. 1. Switzerland’s Swissair has a similar policy.

Air France and Croatia Airlines also require surgical masks.

To date, no U.S. carrier has required passengers to forgo fabric masks. But as the Delta variant continues to spread and other mutated variants raise concerns, policies could change.

Cloth masks have been hot sellers during the pandemic, giving people a way to express themselves through fashion while complying with local and federal mask mandates. They were also easier to find when surgical masks were supply-constrained and being prioritized for medical personnel. Cloth is generally not as effective as other filters.

“The filtration effectiveness of cloth masks is generally lower than that of medical masks and respirators; however, cloth masks may provide some protection if well designed and used correctly,” the Centers for Disease Control and Prevention said last year.

U.S. carriers do have some fabric limitations in place. Virtually all prohibit bandannas and scarves to be used as masks. Ski masks and balaclavas are also not accepted. Currently, masks are required in airports, airplanes, and other forms of public transportation until at least January 2022.

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