Amid the hourly updates on the new coronavirus, a single, calming fact stands out: a particle of happy news, hanging in a cloud of dread. The germ that causes Covid-19 may be responsible for a terrifying public-health disaster, but hallelujah, thank the lord, at least it isn’t airborne.
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 rema
