The U.S. Centers for Disease Control and Prevention (CDC) updated its mask-wearing advice over the weekend to clarify that some masks are more effective at preventing the spread of COVID than others and that people should “wear the most protective mask you can that fits well.”

The agency also removed concerns relating to supply shortages of N95 masks—though it continues to advise that N95 masks labelled “surgical” should be reserved for healthcare personnel use.

On its “Types of Masks and Respirators” page, the CDC highlights the importance of wearing a mask that fits well and does not have any gaps along the edges or around the nose.

It also states loosely woven cloth masks provide the lowest level of protection and that respirator masks that are approved by the National Institute for Occupational Safety & Health (NIOSH) provide the highest level of protection from particles including the virus that causes COVID.

“Loosely woven cloth products provide the least protection, layered finely woven products offer more protection, well-fitting disposable surgical masks and KN95s offer even more protection, and well-fitting NIOSH-approved respirators (including N95s) offer the highest level of protection,” the CDC states.

It comes after health experts have increasingly called on people to upgrade their face masks in light of the highly infectious Omicron variant. Some have gone so far as to say that cloth masks are not good enough.

Do Cloth Masks Work?

This question has been a topic of debate throughout the pandemic. With the emergence of new variants, many scientists are saying cloth masks do not work very well.

“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, told news agency NPR in December.

Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health, told CNN that same month: “Cloth masks are little more than facial decorations. There’s no place for them in light of Omicron.”

While some studies have shown that cloth masks are able to reduce the number of oral contaminants spread by people, not all viruses and bacteria are the same size, and neither are all cloth masks.

One study from February 2021 set out to compare how well cloth face masks blocked aerosol particles from hitting a collection chamber and found that their filtration efficiencies varied massively from as low as 1.4 percent to 98 percent.

Another study from January 2021 recommended the adoption of cloth masks in light of medical mask shortages at that time and noted that they can provide “good fit and filtration for personal protective equipment in some community contexts” but that results would depend on material, design, and how they are used.

In short, it’s hard to say definitively how well cloth masks work, but the consensus from multiple experts is that better-quality masks like N95s are the way to go in light of newer highly infectious COVID variants.

In any case, the CDC, despite its updated guidance, stresses that “any mask is better than no mask” and states that people should wear cloth masks with a proper fit, a nose wire, and multiple layers of tightly woven breathable fabric that block light when held up to a light source.

It also states that people can wear two masks—a disposable mask underneath and a cloth mask on top—for added protection.

Masks should not be worn by children under 2 years of age or people who cannot take them off without help, according to KidsHealth.org.

Vermont Senator Bernie Sanders recently called on the federal government to provide N95 masks to all U.S. citizens.