The friendly skies are becoming not-so-friendly to passengers wearing cloth masks. Several airlines overseas are already banning cloth masks and requiring that travelers wear surgical masks, FFP2 masks, KN95 or N95 masks without valves on flights.

According to Fortune, Finnish airline Finnair and Germany’s Lufthansa are two of the many European carriers shunning cloth masks. So far, no American airline has mandated the more efficient face coverings but with the Delta variant spreading, the policies may change.

While cloth masks have been best-sellers during the pandemic, allowing people to make a fashion or even political statement with their masks, the Centers for Disease Control and Prevention say they are not as protective 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,” said the CDC in a statement last year.

When wearing a cloth mask, two layers are better than one. According to Business Insider, research has shown that that a two-layered fabric mask is superior to a single-layered cloth face covering, so doubling up is recommended. Experts even suggest using three layers to help filter out potentially infectious respiratory droplets and aerosols. A  study in April determined  that when two layers of 600 thread-count cotton were added to a layer of another common fabric such as silk or chiffon, the mask was even more efficient in filtering out particles. However, the study authors warned that if any mask is ill-fitting and has gaps, this can result in a 60% decrease in filtration efficiency.

Experts have noted that it is important to don a good quality mask to stop 99% of virus-laded droplets from being transmitted into the air, according to the New York Daily News. Researchers from Duke University found that the best protection was offered by the N95 masks without a valve, followed by three layered surgical masks.

U.S. airlines do have some mask restrictions, says Fortune. Bandanas and scarves are not acceptable forms of masks, nor are ski masks and balaclavas. Masks are required in all airports, airplanes, and other forms of public transportation until at least January 2022.