They Say Coronavirus Isn't Airborne—but It's Definitely Borne By Air
https://www.wired.com/story/they-say-coronavirus-isnt-airborne-but-its-definitely-borne-by-air/
The word “airborne” means different things to different scientists, and that confusion needs to be addressed.
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The 5-micron cutoff is arbitrary and ill-advised
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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 , 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.
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.
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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.”
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.
