The World Health Organization recently admitted that Coronavirus is airborne and has updated its public health guidance with the following statement on how the SARS-CoV-2 virus is transmitted:
"A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth. The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range)."
WHO's statement explains why it ended-up being wrong about how Coronavirus spreads: it distinguished between 'droplets' and 'aerosols' then assumed only what WHO considers to be an aerosol can fly far enough to be called 'airborne'.
At first the distinction between droplets and aerosols seems clear-cut. Droplets are like cannonballs fired by a cough or sneeze that fall to the ground within 1-2 metres, whereas an aerosol (airborne solution) is made up of respiratory particles that are released when you speak or breathe, and can linger in the air like smoke for several hours.
If SARS-CoV-2 only spread via droplets, interventions that reduced the risk of catching and transmitting the disease, like social distancing — through the two-metre (six-foot) rule — and washing hands could have been enough to slow the spread of the Covid-19 pandemic. But if the virus were transmitted via aerosols, face masks would be needed too. And as we now know, masks are necessary.
Scientific Difference
Both droplets and aerosol particles consist of microscopic spheres of liquid, and scientists distinguish between them based on the size of those tiny globules.
Droplets and respiratory particles are many times larger than SARS-CoV-2. Each virus particle, technically known as a 'virion', is 0.1 microns wide (0.1 μm or 0.0001 mm) and the distance it travels — before it disintegrates or settles on a surface — is influenced by the size of its surrounding globule.
Making a distinction between droplets and aerosols is essential for experiments as researchers must control as many variables as possible to measure the effect of changing one. But results from a laboratory don't always apply in real-world environments, where aerodynamic behavior is affected by factors like air flow from natural ventilation, which may help a 'droplet' float more than 2 metres.
Describing a virus as airborne according to the size of a globule suggests a level of precision that simply isn't relevant outside the carefully-controlled conditions of a lab. So while there's a technically a difference between droplets and aerosol particles, practically speaking it might not matter.
In relative terms, droplets are big and aerosol particles are small. But in absolute terms, the line between the two is determined by an arbitrary threshold.
Historically, health authorities like WHO and the US Centers for Disease Control have used a cut-off of 5 microns (the width of E. coli bacteria) to separate droplets from aerosols.
But many researchers now believe that limit is too low, as globules 100 microns wide can stay airborne for more than 2 metres. In other words, aerosol particles are 20 times larger than previously thought — and shouldn't be called droplets.
Where did the 5-micron figure come from? The source of the error was revealed in an as-yet unpublished study led by fluid dynamics researcher Lydia Bourouiba and history professor Thomas Ewing, thanks to the detective work of Ewing's graduate student, Katherine Randall.
That 'scientific screwup' was recently covered in Wired magazine. According to the story, it originates from the mid-20th century. After experiments by sanitary engineer William Firth Wells found that Tuberculosis is transmitted by particles under 5 microns, CDC scientists interpreted Wells' results out of context and generalized them into an assumption for how all airborne diseases spread.
Wired's article interviewed environmental engineer Linsey Marr, one of many experts who believe WHO didn't acknowledge airborne transmission for historical reasons.
According to Marr's first-hand account of an online meeting between WHO and independent researchers, the organization reacted badly to the suggestion that it was using an outdated threshold to dictate health policy. At one point during a Zoom call, when atmospheric physicist Lidia Morawska was explaining how far particles can potentially travel, one of WHO's advisors cut her off and claimed she was wrong.
That an advisor believed they knew more about aerosols than Morawska — one of the leading experts on aerosol physics — is astounding. The anecdote highlights the conceited attitude of WHO staff, who seem to rely on accepted 'wisdom' over scientific evidence.
But ignoring science isn't the only reason why WHO got it wrong about Coronavirus.
There's a bigger issue caused by defining 'airborne' according to the size of globules: it's based on a belief that public health guidance must revolve around a technical point.
Popular Definition
The problem with saying that only aerosols are 'airborne' contradicts the word's meaning for many people.
To the average person, an airborne virus isn't defined by whether it can travel further than 1 metre, it's a vague idea that a germ hangs around in the air for a short amount of time.
Airborne has a meaning that's already understood by the general population and widely used in everyday language. According to dictionary.com, for instance, that popular definition is "carried by the air, as pollen or dust."
Note that both a pollen grain and dust particle can be larger than 5 microns. The pollen grains released by wind-pollinated plants are 17-58 μm while, according to a WHO document, "dusts are solid particles, ranging in size from below 1 µm up to at least 100 µm, which may be or become airborne".
Trying to reconcile the contradiction between two definitions — scientific and popular — leads to semantic nonsense. You end-up with news headlines that sit on the fence ("Coronavirus Isn't Airborne - But It's Definitely Borne By Air") and clumsy workarounds like 'microdroplets' (still droplets, just smaller!)
Redefining a common term is doomed to fail because it follows two misguided beliefs: that a word should have a single 'correct' meaning; and that you can control how people speak. But as linguists say, language is always changing.
Science Communication
WHO also made the mistake of trying to teach people the scientific definition even though 'airborne' shouldn't be a technical term that requires an explanation.
In July 2020, Nyka Alexander from WHO's communications team had the unenviable task of explaining what scientists call 'airborne' to the BBC. Although she did a decent job based on what WHO's advisors believed, she didn't mention that 241 experts disagreed with that belief.
The organization approached science communication as if it were writing a textbook, by first trying to explain the basics — a scientific definition of 'airborne' — in the hope that everyone can start on the same page. That won't work for health advice, however, because the general public isn't a captive audience of students. In reality, a layperson will stop reading as soon as they lose interest or get confused by jargon.
WHO's approach to communication has cost lives. When providing guidance so that people could protect themselves against Covid, it failed in its mission to "promote health, keep the world safe, and serve the vulnerable" because the organization lost sight of who needs such vital information.
Unlike science education, which usually means teaching new things, science communication often involves helping people understand something through what they already know, like an analogy.
The goal is not to educate, but to communicate. WHO failed at communicating how Coronavirus spreads because it forgot the target audience for public health guidance: the public.
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May 29, 2021 at 10:10AM
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