They Say Coronavirus Isn’t Airborne–but It’s Definitely Borne By Air

This message is now dogma for news outlets and public health officials. They impress on us that droplets laced with the new coronavirus don’t remain aloft for long–that they only sail for six feet at the most before they fall onto the ground. That’s why we’re told that soap and water are the best protections one can find: 20 seconds’ worth of hand-related hygiene, repeated many times throughout the day. The virus isn’t airborne; so keep on washing when you can. The virus isn’t airborne; so you’d be wise to trade your grubby handshake for an elbow bump. The virus isn’t airborne; so don’t forget to keep your fingers off your face.

But I’m afraid this standard line–this single, calming fact about the new coronavirus–may not be as simple as it seems. When health officials say the pathogen isn’t “airborne,” they’re relying on a narrow definition of the term, and one that’s been disputed by some leading scholars of viral transmission through the air. If these scholars’ fears bear out–if the new coronavirus does, in fact, have the potential to travel farther through the air than officials have been saying–then we might need to reevaluate our standards for protecting healthcare workers at the front lines of fighting Covid-19. In fact, we might need to make some tweaks to all our public-health advice.

From early on, any spread of the new virus through the air has been downplayed from the top. World Health Organization director-general Tedros Adhanom Ghebreyesus assured people on Twitter last week that “actually it’s not airborne.” He went on to clarify that “[i]t spreads from person to person through small droplets from the nose or mouth which are spread when a person with #COVID19 coughs or exhales.” According to this way of thinking, the blobs of viral particles that get expelled from coughs and exhales are too big to float around; so they mainly cause infection by landing onto someone close, or by dropping on a surface from which they’re later transferred to someone’s body via touch.

For public health officials such as Tedros (who goes by his first name), a truly airborne virus is one that floats around for extended periods–like measles, which is known to be infectious in the air for at least half an hour. A pathogen like this can create a nightmare scenario. A sick person might ride an elevator, for instance, and shed some virus along the way. Later on, someone else who got into the same elevator might breathe in those germs and develop the disease.

There are very good reasons to believe–and good reasons for public-health officials to assure the public–that the new coronavirus virus isn’t “airborne” in that specific and apocalyptic sense. But the definition used by these officials may also be obscuring vital details of transmission. In particular, it papers over all the nuances in how someone’s virus-laden cough or sneeze or breath really travels through the air. The authorities employ a rule of thumb for distinguishing what they call “droplets” from “aerosols.” Droplets are often defined as being larger than 5 microns in diameter, and forming a direct spray that is propelled by cough or sneeze up to 2 meters away from the source patient. Aerosols, in this scenario, are smaller gobs of potentially biohazardous material that may remain afloat for longer distances.

This black-and-white division between droplets and aerosols doesn’t sit well with researchers who spend their lives studying the intricate patterns of airborne viral transmission. The 5-micron cutoff is arbitrary and ill-advised, according Lydia Bourouiba, whose lab at the Massachusetts Institute of Technology focuses on how fluid dynamics influence the spread of pathogens. “This creates confusion,” she says. First of all, it garbles terminology. Strictly speaking, the aerosols are droplets, too. When you breathe out or cough, you release bits of watery mucus from inside your body in a wide array of sizes, ranging from bigger, wetter ones to finer ones. All of these are droplets. The smallest droplets are commonly described as aerosols. Whatever you call them, though, any of these bits of mucus may be laced with viral pathogens. To make matters more complicated, when the water component of droplets dries up in the air, the remaining bits of floating virus are called “droplet nuclei,” which are even lighter and more apt to travel long distances. Aside from size, other factors, such as local humidity and any drafts of air, will also affect how far a droplet flies.

Even the fattest droplets may not always fall right to the ground within a few feet. When you go to the ocean on a windy day and feel the sea spray on your face, you’ve just encountered droplets of a size that might be described as “not airborne” in a public-health briefing. Even breezes that are far more subtle than the ones coming off the ocean can lift and push a droplet. Oddly though, many traditional studies of droplet trajectories have made use of simplified models that don’t account for the gust of air released when a person coughs or sneezes, which gives those droplets an extra push. Bourouiba calls this a mistake. Her lab has found that coughs and sneezes, which they call “violent expiratory events,” force out a cloud of air that carries droplets of various sizes much further than they would go otherwise. Whereas previous modeling might have suggested that 5-micron droplets can travel only a meter or two–as we’ve heard about the new coronavirus–her work suggests these same droplets can travel up to 8 meters when taking into account the gaseous form of a cough.

For researchers like Bourouiba, who study the physics of pathogens’ paths, any virus traveling in the air might as well be described as “airborne.” But there is no consensus among scientists as to which pathogens should get that label and which shouldn’t. Julian Tang, a virologist at the University of Leicester in England, coauthored a review article on this very topic last year. The paper noted that for some researchers, “airborne transmission” involves only fine aerosols. For others, it can involve both aerosols and larger droplets. Ultimately, in their paper, Tang and his colleagues settled on using the phrase to mean transmission by particles of fewer than 10 microns in diameter–a cutoff twice as large as what WHO has used.

The debate over whether something is “airborne” is particularly sensitive around pathogens that cause the most acute, deadliest outbreaks. But there’s not even agreement among experts as to how regular old influenza transmits through the air. Those who say the flu does this well point to a curious incident from the 1970s in which an airplane with 54 passengers was grounded on the tarmac for three hours because of engine issues during a takeoff attempt. There was one person who had been ill onboard; and within three days, three-quarters of the other people who had been on the plane showed symptoms of flu such as cough, fever and fatigue. The majority of those tested were positive for the virus. Donald Milton, whose research at the University of Maryland School of Public Health includes studies of infectious bioaerosols, says that all these years later he and his peers are still trying to convince other scientists that influenza is substantially airborne. He published a paper in 2018 asserting that, contrary to what some might think, sneezing and coughing are not required for influenza virus to be released in an aerosol form that can float around.

Meanwhile, the aerodynamics of more exotic pathogens have stirred controversy. One infectious disease expert warned, in 2014, that Ebola might become highly transmissible by air. This proved to be a false alarm. There is some evidence that coronaviruses such as SARS and MERS can travel in hospital air. Some researchers still dispute these data: the MERS research, for example, did not use a hospital room without infectious patients as a control. But others take it as a given that these coronaviruses were floating in their infectious form around parts of hospitals.

As for the airborne behavior of the new coronavirus, scientists are racing to obtain data. A study published in the Journal of the American Medical Association on Mar. 4 looked at the hospital isolation rooms of three patients in Singapore with Covid-19. The study offered some solace because it didn’t find evidence of the virus in air samples. However, the air vent blades in one patient’s room did test positive. A second study, described in a preprint paper published on Mar. 10, examined the hospital environments of Covid-19 patients in Wuhan, China. Although the levels of the microbe that causes Covid-19 in most rooms were undetectable or low, the study did find the presence of the virus in aerosol form. That there would be non-negligible amounts of virus in the air does not surprise Linsey Marr, a researcher at Virginia Tech who studies the dynamics of viruses in the air. “This is exactly what I suspected,” she says. Even before that paper came out, she’d told me it’s “unfortunate” that the WHO insists on saying that the new coronavirus “is not airborne.”

Crucially, the hospital studies only looked for the genetic signature of the virus, as opposed to mixing the viral material with animal cells to see whether it would wreak havoc. As such they could not know whether the viral material present in the ventilation system or the air was infectious. This is a critical point–virologists emphasize that the presence of residual RNA or DNA left by pathogens in no way guarantees that people might get sick from it. However, the question of whether the new coronavirus is infectious as an aerosol was explored in another paper posted as a preprint this week. In that study, scientists used a laboratory machine to force the virus into aerosolized form and then tracked it for 3 hours. They found the pathogen was still able to infect animal cells at the end of that timeframe, although there was substantially less of virus suspended in the air from one hour to the next.

These three new papers should not be overinterpreted. Only one of them has been vetted by peer review at this point. It also remains unclear, and undemonstrated, whether the Covid-19 virus released from patients’ lungs comes out in aerosol form; whether aerosolized particles of this virus travel significant distances; and, if so, whether they do so in sufficient number to cause infection. Notably, while the joint WHO-China mission report published in late-February said that although airborne particles were “not believed to be a major driver of transmission,” it noted that such a mode “can be envisaged if certain aerosol-generating procedures are conducted in health care facilities.”

Given that much research on airborne transmission in outbreaks is focused on medical settings, it’s also less than clear how even the most common viruses might pass from person to person under everyday circumstances. Julian Tang and his colleagues have created a visualization of the breaths exchanged by two people in conversation standing three feet apart. Most of the time, the puffs of air they let out remain separate; but portions of their exhalations do sneak from each person’s breathing space into the other’s. Given all this uncertainty, some experts say there needs to be better public messaging on the spread of the new coronavirus. “Crowded public transport where people can breathe on each other may also lead to transmission of infection,” Tang says, echoing public-health advice that, while widespread, may not be getting as much emphasis as hand-washing. Milton agrees, adding that it might be wise to shut off air-recirculation systems in cars, which could potentially spread the pathogen among passengers.