Since the beginning of the pandemic, authoritative sources have stressed that SARS-CoV-2, the novel coronavirus, spreads through very close personal contact. For Vox, Journalist Brian Resnick explains the understanding that the World Health Organization (WHO) originally held:
The virus-laden droplets exhaled from a sick person’s mouth and nose, the thinking goes, are heavy, and fall to the ground before they can get much farther than 6 feet.
Then other clues began to show up, like a practice session where 52 healthy people and one suspected “superspreader” walked into a room, and 53 infected people walked out. These choir members may have shaken hands and even briefly hugged, but probably had no closer contact. Chances are, the problem lay not in brief personal interchanges, but in the cloud of aerosol droplets exuded by one person, that gradually infiltrated all the air in the enclosed space.
For some arcane traditional professional reason, many scientists have been willing to accept the “airborne” designation for only a strictly delineated class of illnesses. Finally, in mid-August, after receiving a letter of concern signed by 239 scientists and engineers, the WHO “changed its language to recognize that fact” and hopefully, to impress upon the world that the ventilation of indoor spaces is key.
Resnick’s article goes into much finer detail on how the droplet versus aerosol debate powered this controversy. The new understanding is explained in a basic way by MIT researcher Lydia Bourouiba, whom he quotes:
We’re always exhaling, in fact, a gas cloud that contains within it a continuum spectrum of droplet sizes. The cloud mixture, not the drop sizes, determines the initial range of the drops and their fate in indoor environments.
The people who produce these clouds do not even need to sing or shout. Normal conversation will do it. The behavior of any individual droplet is dependent on its contents (other than virus organisms), the environment’s temperature and humidity, and how fast the cloud travels.
Apparently, the outer layers of the cloud serve to coddle and protect a certain proportion of the aerosol droplets, enabling their ambitious journeys and influencing the length of time they remain viable. On the other hand,
Just because a virus travels far in a drop doesn’t mean it can infect people across great distances. Viruses can degrade quickly outside the body. Also, dose matters. Small exposures to the virus may not be enough to get a person sick.
Shockingly, “the virus can live in an aerosolized form for up to 16 hours.” But that does not necessarily mean it always does. The cause is, like so many other causes, multifactorial, and the risk is not an either/or proposition, but a spectrum.
One reason why people in the profession are picky about rigid categories regarding the definition and degree of aerosolization is that…
[…] there are very specific sets of guidelines in place to deal with extremely contagious airborne diseases in a hospital setting… “Airborne” means something very specific, very resource-intensive, and very scary for hospitals and the people who work in them. And Covid-19 doesn’t match that definition.
Transferring any of this knowledge from acute hospital and lab situations, to figuring out what to do about the schools to which children either do or not return, is making everybody crazy.
Your responses and feedback are welcome!
Source: “The debate over ‘airborne’ coronavirus spread, explained,” Vox.com, 07/13/20
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