https://www.wired.com/story/they-say-coronavirus-isnt-airborne-but-its-definitely-borne-by-air/
Aerosol through breathing distribution video :
https://media.wired.com/clips/5e6a65157c07c90008d05257/360p/pass/PLoS-video-V3.mp4
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.
Citat:
The research also found that when the virus was artificially turned into an aerosol, it remained viable for three hours.
The virus becomes an aerosol when particles remain suspended in the air as tiny droplets — five microns in diameter or less — much smaller than the majority of the droplets you generate when you sneeze or cough.
"Once it gets quite small like that then it basically stays aloft, it's like cigarette smoke," Dr Tovey said.
