Experts can't agree on how long the coronavirus lingers in the air, and the controversy has huge implications for mask-wearing

• The CDC recommends people in the US wear masks when they must venture out into public places right now, in case they are infected with COVID-19.
• But the WHO stresses that there's "no evidence" healthy people wearing masks will do much to stop a pandemic.
• The disagreement underscores how tricky it is to pin down how infectious diseases are transmitted between people. Does the coronavirus linger in the air, or not?
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As the coronavirus takes a deadly toll across the US and UK this week, public health experts are struggling to uniformly answer a seemingly straightforward question: Should we all be wearing masks?

In the US, the Centers for Disease Control and Prevention suggests it's probably a good idea to cover up with a homemade face covering if you must be out in public.

But the World Health Organization isn't so sure.

"There is currently no evidence that wearing a mask (whether medical or other types) by healthy persons in the wider community setting, including universal community masking, can prevent them from infection with respiratory viruses, including COVID-19," the global public health arm of the United Nations said in new guidance released this week.

The disagreement hinges, in part, on a question scientists don't yet have clear answers for: How far can coronavirus droplets travel from an infected person, and how long can they stay aloft in the air?

The World Health Organization contends that the coronavirus spreads by large droplets that get expelled into the air when sick people spit, talk, cough, breathe, sing, or sneeze. Though they are thousands of times smaller than a plump drop of rainwater, these droplets are thought to be too big and heavy to stay aloft for very long after a person expels them.

This, the WHO stresses, means the likelihood the coronavirus is airborne and lingering in the atmosphere for any sustained period of time, is very low.

Most coronavirus cases are spread through human-to-human contact

"When you look at the sheer number of positive cases, they're happening with very clear mixing and mingling, and they're very close with each other," WHO assistant director-general for antimicrobial resistance Dr. Hanan Balkhy told NPR recently. "So that does not indicate airborne transmission."

This is one of the key reasons the WHO says you're probably only going to catch the virus by coming in contact with a sick person, or by touching a contaminated surface and then putting your hands to your eyes, nose, or mouth.

"There is limited evidence that wearing a medical mask by healthy individuals in the households or among contacts of a sick patient, or among attendees of mass gatherings may be beneficial as a preventive measure," the latest WHO guidance on masks reads.

Don Milton, a virologist at the University of Maryland who studies how people catch and transmit viruses, believes the WHO's guidance on airborne transmission doesn't follow the rules of "good science."

"It's ridiculous. They don't know. They don't know!" he said of the droplet-versus-aerosol debate. "It's very hard to separate those modes out. You know, people have been trying for flu for years, and we still haven't been able to prove one way or the other what route flu is transmitted. So how, after three months, do we know this for COVID-19?"

It's possible that the virus might linger in the air

Like Milton, the CDC is more cautious about how the virus spreads, suggesting aerosolization might occur, especially in healthcare settings.

"The contribution of small respirable particles, sometimes called aerosols or droplet nuclei, to close proximity transmission is currently uncertain," the CDC says in its COVID-19 guidance to healthcare workers for infection control. "Airborne transmission from person-to-person over long distances is unlikely."

This is why the CDC cautions that "procedures that are likely to induce coughing" like suctioning of patient airways "should be performed cautiously, and avoided if possible," at the hospital right now, because there's the potential for coronavirus transmission in the air in such spaces.

"There's no exact cutoff between a large droplet and an aerosol, it's a continuum; as they get smaller, they stay in the air longer, and they have the potential to travel farther," Milton said. "We like to have things in neat little categories, but life doesn't really work like that. Those are just artificial boundaries we impose to try to understand what's going on and it doesn't, in this case, help a whole lot."

Masks can work when used by sick people and healthcare workers

There are, however, two key points of agreement among public health experts about masks.

First, masks do help if you put them on sick people. The problem with COVID-19, though, is that patients may not know they are sick, either because they're pre-symptomatic and are still feeling well, or because they're asymptomatic and may never feel ill. These people can still infect others, though, if they share the virus through droplets.

"There's a very common view that we're wearing masks not only to protect ourselves, but because if we have been infected, we're then protecting other people by wearing the mask," Ben Cowling, a professor of epidemiology and a mask researcher at the University of Hong Kong's School of Public Health, recently told Business Insider.

One recent study in Singapore, where coronavirus patients and their contacts were tightly tracked, found that more than 6% of coronavirus cases acquired there likely came from people who were sick - but before they ever displayed symptoms.

In Iceland, where 5% of the population has been tested for the coronavirus, roughly half of those who tested positive were completely asymptomatic, as CNN recently reported.

Dr. Anthony Fauci, the US's leading public health voice in this outbreak, says "right now we're just guessing" about how many asymptomatic coronavirus carriers might be in the general population, but he suggested recently that "somewhere between 25 and 50%" of COVID-19 cases may be unknowingly carrying and potentially transmitting the virus to others.

The second place masks are imperative is in hospitals, since they have lots of sick patients. Here, face masks, along with eye shields, gowns, and gloves, are important pieces of protective equipment for doctors and healthcare workers who labor in environments where coronavirus particles surround them all day long.

Healthcare workers shouldn't wear homemade masks

There's good evidence that homemade masks should be an absolute "last resort" in these settings, as they don't provide nearly as much protection as surgical masks, which are made from fabrics designed to trap viruses.

The WHO recommends N95 masks for healthcare workers who need to intubate, ventilate, or resuscitate patients - or do any other "aerosol-generating procedures." Other researchers suggest that if healthcare professionals must use reusable, homemade masks (because manufactured masks aren't available) that they wash and thoroughly dry them often, and put them on and take them off carefully, without touching the outside of the mask.

It may be years or even decades before we understand more fully how this new virus operates, and how much wearing masks in public can really help stop the spread. It took seven years at one Baltimore VA hospital before researchers finally showed conclusively that tuberculosis could - occasionally - be transmitted through the air, without any face-to-face contact (in a study performed between people and guinea pigs).

This is why Milton argues there's a lot we have to learn about this virus, and how it's really transmitted.

"The problem is that with clinical epidemiology, you know that people have been in close contact, and that's when transmissions happened, but you don't know what happened when they're in close contact," he said. "You don't know whether they actually touched, you don't know whether it was large droplets, or whether they were just close enough that they breathed a lot of the other person's exhaled breaths. You can't tell a difference."

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