Dr. Jay Varma, City Hall’s top health expert on public health, is advising New Yorkers to consider wearing two masks instead of one due to the more highly-contagious variants of coronavirus circulating around the world.

“I am now starting to wear a surgical mask covered by a fabric mask,” Dr. Varma said at a Tuesday morning briefing with Mayor Bill de Blasio. “That is really the current thinking that we have right now—that more is probably better, even though we don't know for sure why these new strains are more infectious.”

The use of more than one mask—often with a cloth covering over a surgical mask—has come into vogue over the last month after the first variant of concern was announced in the United Kingdom. Two concerning versions of the virus have also been found in Brazil and South Africa So far, the Centers for Disease Control and Prevention are reporting 293 cases of the U.K. variant--with 22 detected in New York. Minnesota reported the first U.S. case of Brazil’s variant on Monday, and no states have spotted the South African variant though experts believe it is here.

The Centers for Disease Control and Prevention has yet to recommend double-masking, but Dr. Anthony Fauci said that double-masks are okay as a common sense method in warding off infection. Martin Fischer, a professor of chemistry at Duke University, agrees.

“For many household masks, such as thin cotton masks, the protection can increase when doubled up,” Fischer said. “However, there might be occasions where that is not true, such as when the resulting doubled mask is so dense that breath escapes more through gaps, rather than through the mask, or when wearing a well-fitting mask over an ill-fitting mask makes the fit worse.

Monica Gandhi, professor of medicine and an infectious disease expert at the University of California, told The Washington Post that two layers of masks should be worn in crowded places. She said doing so won’t obstruct breathing and could offer protection on par with N95 masks, the high-grade coverings worn by medical professionals.

Fischer added that N95 masks are well-fitting and valveless, so they don’t need a second layer.

Dr. Varma said stricter guidelines on mask wearing are expected soon.