As new COVID-19 variants spread across the country, health authorities are once again guiding people to stay indoors and mask up. That said, they’re now urging folks to wear N95 or similar high-filtration masks when in public indoor spaces.

“Cloth masks are not going to cut it with omicron,” Linsey Marr, a researcher at Virginia Tech who studies how viruses transmit in the air, said, according to NPR.

Data from other countries reveal the omicron variant is much more transmissible than previous coronavirus variants with one person infecting at least three others at a time on average.

“It’s very contagious,” says Dr. Robert Wachter, chair of the Department of Medicine at the University of California, San Francisco, according to NPR. “And the kind of encounter that you could have had with prior versions of the virus that would have left you uninfected, there’s now a good chance you will get infected from it.”

Marr notes that a cloth mask can be a “marginally OK to maybe a decent filter,” however she says that in times like this, with more transmissible variants, just “OK” isn’t enough.

Data from scientists at the University of Hong Kong shows that omicron multiples 70 times faster inside human respiratory tract tissue than the delta variant does. The study also reveals that omicron reaches high levels in respiratory tract tissue 48 hours after infection, compared to the delta Variant.

“That would suggest to me that maybe it reaches higher levels and then we spew out more [virus particles] if we’re infected,” Marr says.

Dr. Abraar Karan, a Stanford University infectious disease physician, says virus particles from an infectious person can linger in the air indoors for minutes or hours after they leave a room in some situations.

“I think that people need to realize that transmission here can happen even when you’re not near somebody,” he says.

N95, KN95 and KF94 respirators are all made out of material with electrostatic charge, which “actually pulls these particles in as they’re floating around and prevents you from inhaling those particles,” Karan notes. “And that really is key.” That’s because if you don’t inhale virus particles, they can’t multiply in your respiratory tract.