As COVID-19 cases increase sharply nationwide, some health experts are now recommending that seemingly healthy members of the public wear cloth masks when they’re out and about. On April 3, President Trump announced a new federal recommendation urging the public to wear cloth masks to prevent people who are infected, but may not have symptoms, from unknowingly spreading the disease.

The recommendation is an about-face from previous guidance on mask usage. Until now, officials at the World Health Organization, the US Centers for Disease Control and Prevention, and other agencies worldwide have discouraged the public from wearing masks unless they are sick or caring for someone who is sick. They noted that there is little evidence to support mass masking and that the limited data we do have suggests it may reduce disease transmission only marginally at best.

With evidence of benefits in short supply, experts also raised concerns about potential harms. Mask wearing may give people a false sense of security, some experts said. This may lead some members of the public to be lax about other, far more critical precautions, such as staying two meters apart from others, limiting outings, and washing their hands frequently and thoroughly.

Moreover, donning an uncomfortable, awkward mask may lead some people to touch their faces more, some argued. Any face touching has the potential to transfer virus particles from contaminated hands to entry points, such as the eyes, nose, and mouth. And even if a mask-wearer’s hands are clean to begin with, simply touching their mask could contaminate their hands if there are viral particles caught on the outside. If that’s the case, a mask wearer could then transfer virus particles from their mask to their face unwittingly—negating any benefit of having the mask. They might also transfer the virus from their mask to their environment by touching surfaces, where the virus particles could get picked up by other people.

Last, they argued, the masks that would be most effective at stopping the new coronavirus—SARS-CoV-2—are things like N95 respirators and surgical masks, which are in short supply worldwide. Without question, these should be preserved for the heroic frontline health workers, who are putting their lives at risk every day to treat patients with COVID-19 during this overwhelming pandemic.

While experts unanimously agree on that last point—that proper medical masks should go to healthcare workers first—the other points are now up for debate.

With SARS-CoV-2 now spreading widely and unchecked through communities nationwide, experts are taking a more favorable look at the limited data behind masks preventing disease spread among the public. In a recent a commentary in The Lancet, a group of UK and Hong Kong researchers argue that “there is an essential distinction between absence of evidence and evidence of absence.” And though there are no large, high-quality studies looking at public mask usage, there is some data to support mask usage.

As for potential harms of mask wearing—such as fraught face fussing and wearers relaxing other precautions—experts are now dismissing the concerns. Wearing a mask in public could keep people alert to current health risks in public, some experts say. A conspicuous mask strapped to your head is a constant reminder right over one’s nose to be mindful of possible viral transmission. And—as a bonus—if everyone wears a mask, it could lessen the chances of stigma of those who wear them because they are sick.

Of course, wearing a mask does not replace other interventions, like staying two meters apart or practicing good hand hygiene, says Joseph Allen, an expert of exposure assessment science in the Department of Environmental Health at Harvard T.H. Chan School of Public Health. “It’s just one more layer of protection.”

“The scale and scope of what we’re facing requires or mandates taking every precaution we can,” he says.

As such, some experts—Dr. Allen included—are now in favor of having the healthy public wear home-made cotton masks or other face coverings that could act as a basic physical barrier.

“It’s not as good [as medical masks], but it’s better than nothing,” Dr. Allen argues.

So with the conflicting reasoning and logic, what does the data on masks actually show? How were they dismissed before but embraced now? Here’s a rundown of pertinent data.

Transmission

First, to understand why masks could be helpful at blocking SARS-CoV-2 transmission, it’s important to understand how the coronavirus is transmitted. And, frankly, we still don’t know all the answers to this question.

So far, SARS-CoV-2 mainly appears to move from one human to another by being launched in relatively large respiratory droplets. These are unleashed when an infected person breathes, talks, coughs, or sneezes. These droplets are relatively heavy, they generally don’t travel much farther than two meters away from their launch site, and they follow a ballistic trajectory, that is, they fall toward the ground after blast off. But if they land on a person’s face before that or fall to a surface a person will soon touch, they could cause a new infection.

There’s also the possibility that SARS-CoV-2 can spread in smaller respiratory droplets called aerosols (less than 5 micrometers). These are droplets we exhale that are so small they can hang in the air for minutes to hours. Experts at the WHO and elsewhere say that the data so far suggests aerosol transmission is not the primary way the virus spreads, though it is still possible. So far, it appears aerosol transmission is mainly a concern for healthcare workers while they’re performing certain medical procedures on COVID-19 patients, such as placing a tube in their airway to aid breathing (intubation). This may create circumstances for the virus to aerosolize and linger in the air in hospital rooms.

Whether SARS-CoV-2 is aerosolized in everyday settings is still unknown and up for debate. Some experts, such as Dr. Allen, believe that it could be happening. Others are more skeptical of the idea given that infected people only infect two to three other people on average. If each COVID-19 patient were creating infectious clouds of SARS-CoV-2 wherever they went, some experts would expect the patients would infect far more people on average. Measles, for instance, has been associated with airborne transmission for decades and each measles patient, on average, may infect 12 to 18 people—or more. There’s even evidence the measles virus can disperse through ventilation systems.

Though that doesn’t appear to be the case for SARS-CoV-2, experts like Allen caution that the size of virus-containing respiratory droplets is a continuum, not subject to clear cut-offs or strict rules. There is evidence that if SARS-CoV-2 does make it into aerosols, the virus can survive in the air for hours in laboratory conditions.

Last, there’s evidence that SARS-CoV-2 particles that land on surfaces or objects can loiter and potentially be picked up by others. These contaminated surfaces and objects that can then transfer the infectious virus particles are called fomites. Masks may keep a wearer from putting a fomite-contaminated hand to their face, but masks may also act as fomites.