New WHO report: defining "airborne" is very, very complicated

May 10, 2024

Do you remember this tweet?

It was March 28th, 2020. COVID-19 was spreading through the United States. Trump had recently declared a national emergency. Most flights from Europe had been stopped.

Everyone was looking to public officials for advice on how to stay safe.

The World Health Organization (WHO) had one overarching job in the midst of this crisis — to educate the public, using the best available scientific knowledge, on how the virus spreads and how to avoid it.

They failed.

Even before any experiments were run and any data collected, science said that SARS-CoV-2 virus should be transmitted through the air considerable distances inside rooms and on trains, buses, and airplanes. Why? Because the COVID-19 virus is nearly identical to SARS-CoV-1, the virus that causes SARS. Just look at these two microscope images:

In 2003 about 8,400 cases of SARS were recorded. A 2004 study found evidence of airborne spread of SARS through a large housing complex. A 2005 study documented airborne spread in a hospital ward between beds. Another study from 2005 used air sampling near the bed of an infected patient to directly measure the spread of the viral RNA through the air via aerosols.

Based on this, and based on experience with other viruses like MERS, RSV, and influenza, in early 2020 most experts believed SARS-CoV-2 should also be able to be transmitted at least several meters through the air.

A research letter published in JAMA on March 4th, 2020 reported on air sampling done in the bedrooms of COVID-19 patients in a Chinese hospital. The air samples were all negative. However, they were careful to note those negative results may have been due to the rapid rate of air exchange in the hospital’s rooms coupled with insufficient air sampling time. The researchers also swabbed around the rooms, finding viral RNA almost everywhere they swabbed, including near the ventilation ducts. This led them to conclude that the virus was being spread considerable distance by respiratory droplets.

Finally, a study published on March 17th, 2020 in the New England Journal of Medicine presented direct experimental evidence that the virus can linger in the air in tiny droplets (aerosols) for hours while remaining viable. Years of research had already established that human speech generates large amounts of aerosols. It doesn’t take a Ph.D. to realize this is a recipe for long-distance spread through the air.

(Post-publication addendum): There was also a study from March 10, 2020 by researchers in Wuhan that provided additional evidence of spread via aerosols. They suggested that the redoning of masks creates aerosols laced with SARS-CoV-2.

The WHO decided not to relay any of this critical information to the public. Instead they decided to make a definitive announcement that “COVID-19 is NOT airborne”, even capitalizing the word “not” for emphasis.

To avoid COVID-19, according to the WHO, you just had to stay greater than 1 meter away from infected people and avoid contact with contaminated surfaces.

The disaster that issued from this science communication failure is hard to wrap one’s mind around.

How much time, energy, and money was lost sanitizing surfaces? How many people decided not to mask or social distance? How many hospitals and schools did not invest in air filtration? How many people died as a result?

It took until October 2020 for the WHO to acknowledge that COVID-19 can be carried through the air more than 1 meter. Even then, it took another six months to a year before they had updated all of the advice on their website to reflect the reality of airborne transmission.

The WHO’s new attempt to deflect blame

About a week and a half ago the WHO released a 52 page report entitled “Global technical consultation report on proposed terminology for pathogens that transmit through the air”. The stated purpose of the report is to define what is meant by the term “airborne”. According to the report, this is a task of “enormous complexity”:

It’s pretty clear what’s going on here. The WHO is trying to save face by making it seem like the definition of “airborne” is very complicated. They are asking us to cut them some slack for their horrible mistake. In the rest of this post I’ll explain why they don’t deserve any forgiveness or slack for their error.

Everyone knows the definition of “airborne”

As the word itself implies, "airborne" means "carried through the air". Just as how "waterborne" means "carried through water" and "bloodborne" means "carried through blood". Both the Merriam Webster and Oxford dictionaries make this clear.

There is only one major complication that needs addressing — how far does viable virus travel through the air, on average?

Larger droplets fall to the ground faster, so they don’t have as much time to travel, either by diffusion or air currents. So viruses that depend on larger droplets for transmission don’t travel as far through the air.

This is not rocket science. Figure from Morawska and Cao, 2020.

As you can see from the figure below, a 100 micrometer droplet takes 5 seconds on average to fall from 1.5 meters height to the ground, while a 1 micrometer droplet takes 12.2 hours to fall the same distance, on average.

The confusing and unnecessary debate about “droplet” vs “airborne” transmission

The WHO could have said something like “COVID-19 is airborne. Experience with SARS and preliminary data suggests it can be transmitted over distances of five meters or more in a room with an infected person in it”.

Apparently, however, the WHO thought this was far too complicated and nuanced for everyday peons to understand. They wanted to give a simple “YES” or “NO”. I suspect they went with “NO” because they were worried people would lose their minds and panic otherwise. If so that is very ugly elitism, but it would be consistent to similar elitism that we saw at the CDC and FDA during the pandemic.

Of course, WHO officials knew that COVID-19 had to transmitted through the air to some degree. At least 1 mm. So they decided to redefine the word “airborne”.

Following a convention that already existed in some scientific literature, they decided to invoke a distinction between "droplet transmission" and "airborne transmission".

Droplet transmission is when the virus can only travel via heavy droplets that quickly fall to the ground. However, even though the virus is still traveling through the air, droplet transmission is not “airborne transmission”. Instead, the term “airborne transmission” means “travel further distances via smaller droplets (aerosols)”.

Obviously that is very confusing. Where do you draw the line? There is no natural dividing point between the two forms of transmission, as this graphic illustrates:

New terms should invented sparingly, and only when helpful. Arbitrary distinctions should only be invoked when absolutely necessary. Ironically, in what was apparently an attempt to simplify things down to a “NO” on the question of airborne transmission they ended up making things way more confusing than they needed to be.

Whether to invoke this distinction and where to draw the line became the subject of debate among public health experts. Thus we ended up with a parade of articles along the lines of "IS COVID-19 AIRBORNE? SCIENTISTS DEBATE". The articles made it sound like scientists had no idea whether COVID-19 could travel through the air. The actual reality was that almost all disease experts worth their salt believed that COVID-19 could travel at least a few meters through the air. The WHO was singularly out of step with the vast majority of the scientific community.

Measuring how far a virus can be transmitted through the air is very easy

Most of the evidence for the airborne transmission of COVID-19 gathered early on was indirect. However, transmission through the air is very easy to directly quantify. First, have an infected person stand in a room with still air and then have them speak. Alternatively they can lay in a hospital bed. The person will start to generate droplets. Next, wait a period of time for the droplets to dissipate and reach steady state concentration in the air. Then, collect air samples at different distances away from the speaker and test them for viral RNA. You will see the concentration of viral RNA in the air decreases with distance. Finally, you also can try to culture the virus to see if it’s still viable. If you want to prove without a doubt that the virus can infect through the air, you can use animals like guinea pigs (this was done in 1959 to prove that tuberculosis is airborne).

Now, you do need to generate and sample for some time (potentially hours) to get enough copies of virus at the longer distances. On the basis of this, some people say that the experiment is “hard”. But this is still an easy experiment as far as experiments go. I bet you could do this experiment really well for under $10,000.

However, the WHO did not want to run this simple experiment before making a proclamation to the entire world about the airborne transmissibility of COVID-19.

Experiments similar to this started to appear in the literature in mid 2020. However, as far as I can tell there is still no standard experimental framework here similar to how there are standardized ways of measuring other things in medicine like children’s bone development or the amount of calcified plaque in the heart.

Conclusion - #COVIDisAirborne

Shortly after the WHO’s announcement a mountain of evidence started to emerge showing that COVID-19 can travel through the air considerable distances — at the least much more than 1 meter. An April 27, 2020 study documented the spread of COVID-19 through two Chinese hospitals, giving evidence for long-distance aerosol transmission. An August 4, 2020 study reported that air samples 5 m from a hospital bed contained large amounts of virus. By September 2020 there were at least eight studies on air sampling in hospitals and in controlled environments, including some studies that tried to culture virus to see it was viable. The results were not uniform, and one study found no evidence for airborne transmission even at very close ranges. However, when viewed in aggregate the studies pointed to significant transmission potential through the air (at least five meters), especially in rooms with poor ventilation. Today every health authority in the world acknowledges that COVID-19 can be transmitted long distances via aerosols (across rooms, though ventilation ducts, etc).

As a reminder — the next pandemic is coming, it's just a matter of time.. and it could be far more deadly.

COVID-19 was a test for humanity. We did really poorly - either a D or an F.

I hope people learn from all of the failings that happened. At the very least, can we please not try to cover things up by inventing silly excuses for past failings?

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