This article describe's a scientist's attempt to figure out where the 5 micron number, and general belief that most respiratory diseases weren't airborne, came from. She eventually traces it back to a particular number developed for a very different purpose.

 I have not fact checked it extensively, but last winter I did try to look into the general state of knowledge on airborne transmission vs fomites and found it weirdly empty, in ways that are consistent with this article.

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That article is incredible. Quoting the part that specifically discusses where the 5-micron myth came from:


In 1934, Wells and his wife, Mildred Weeks Wells, a physician, analyzed air samples and plotted a curve showing how the opposing forces of gravity and evaporation acted on respiratory particles. The couple’s calculations made it possible to predict the time it would take a particle of a given size to travel from someone’s mouth to the ground. According to them, particles bigger than 100 microns sank within seconds. Smaller particles stayed in the air. Randall paused at the curve they’d drawn. To her, it seemed to foreshadow the idea of a droplet-aerosol dichotomy, but one that should have pivoted around 100 microns, not 5. 

[... A]s spring turned to summer, Randall started to investigate how Wells’ contemporaries perceived him. That’s how she found the writings of Alexander Langmuir, the influential chief epidemiologist of the newly established CDC [around 1950]. Like his peers, Langmuir had been brought up in the Gospel of Personal Cleanliness, an obsession that made handwashing the bedrock of US public health policy. He seemed to view Wells’ ideas about airborne transmission as retrograde, seeing in them a slide back toward an ancient, irrational terror of bad air—the “miasma theory” that had prevailed for centuries. Langmuir dismissed them as little more than “interesting theoretical points.”

But at the same time, Langmuir was growing increasingly preoccupied by the threat of biological warfare. He worried about enemies carpeting US cities in airborne pathogens. In March 1951, just months after the start of the Korean War, Langmuir published a report in which he simultaneously disparaged Wells’ belief in airborne infection and credited his work as being foundational to understanding the physics of airborne infection.

How curious, Randall thought. She kept reading.

In the report, Langmuir cited a few studies from the 1940s looking at the health hazards of working in mines and factories, which showed the mucus of the nose and throat to be exceptionally good at filtering out particles bigger than 5 microns. The smaller ones, however, could slip deep into the lungs and cause irreversible damage. If someone wanted to turn a rare and nasty pathogen into a potent agent of mass infection, Langmuir wrote, the thing to do would be to formulate it into a liquid that could be aerosolized into particles smaller than 5 microns, small enough to bypass the body’s main defenses. Curious indeed. Randall made a note.

When she returned to Wells’ book a few days later, she noticed he too had written about those industrial hygiene studies. They had inspired Wells to investigate what role particle size played in the likelihood of natural respiratory infections. He designed a study using tuberculosis-causing bacteria. The bug was hardy and could be aerosolized, and if it landed in the lungs, it grew into a small lesion. He exposed rabbits to similar doses of the bacteria, pumped into their chambers either as a fine (smaller than 5 microns) or coarse (bigger than 5 microns) mist. The animals that got the fine treatment fell ill, and upon autopsy it was clear their lungs bulged with lesions. The bunnies that received the coarse blast appeared no worse for the wear.

For days, Randall worked like this—going back and forth between Wells and Langmuir, moving forward and backward in time. As she got into Langmuir’s later writings, she observed a shift in his tone. In articles he wrote up until the 1980s, toward the end of his career, he admitted he had been wrong about airborne infection. It was possible.

A big part of what changed Langmuir’s mind was one of Wells’ final studies. Working at a VA hospital in Baltimore, Wells and his collaborators had pumped exhaust air from a tuberculosis ward into the cages of about 150 guinea pigs on the building’s top floor. Month after month, a few guinea pigs came down with tuberculosis. Still, public health authorities were skeptical. They complained that the experiment lacked controls. So Wells’ team added another 150 animals, but this time they included UV lights to kill any germs in the air. Those guinea pigs stayed healthy. That was it, the first incontrovertible evidence that a human disease—tuberculosis—could be airborne, and not even the public health big hats could ignore it.  

The groundbreaking results were published in 1962. Wells died in September of the following year. A month later, Langmuir mentioned the late engineer in a speech to public health workers. It was Wells, he said, that they had to thank for illuminating their inadequate response to a growing epidemic of tuberculosis. He emphasized that the problematic particles—the ones they had to worry about—were smaller than 5 microns.

Inside Randall’s head, something snapped into place. She shot forward in time, to that first tuberculosis guidance document where she had started her investigation. She had learned from it that tuberculosis is a curious critter; it can only invade a subset of human cells in the deepest reaches of the lungs. Most bugs are more promiscuous. They can embed in particles of any size and infect cells all along the respiratory tract.

What must have happened, she thought, was that after Wells died, scientists inside the CDC conflated his observations. They plucked the size of the particle that transmits tuberculosis out of context, making 5 microns stand in for a general definition of airborne spread. Wells’ 100-micron threshold got left behind. “You can see that the idea of what is respirable, what stays airborne, and what is infectious are all being flattened into this 5-micron phenomenon,” Randall says. Over time, through blind repetition, the error sank deeper into the medical canon. The CDC did not respond to multiple requests for comment.

If CDC scientists aren't able to get something relatively straightforward like that right, we might also expect them to get other things wrong. One thing that's on my mind is chronic lyme where I'm quite uncertain about who's right. 

It’s a particularly strange mistake to have been enshrined as the CDC surely had TB experts on hand that would have known that it atypically targets smaller lung structures. The failure to integrate their knowledge in policy and official positions indicate a breakdown in the organizational management and knowledge transfer system. 

An interesting followup question to ask as a rationalist may be what were the procedures, methods, and organizational dynamics like, formal or informal, that were operant at the time that led to this ’5 micron’ idea being first put on the record. And to compare to the present day.

I think your alternate universe twitter thread is also worth posting as a comment


From the alternate universe where not all staff of the World Health Organization should be immediately fired:


14 January 2020

GENEVA — In an emergency press briefing by the World Health Organization, WHO Director-General Tedros Adhanom Ghebreyesus announced a "verification crisis" in tracking down sources for the long-standing medical view that airborne transmission is rare among human pathogens. Tedros announced an immediate investigation of the WHO's processes by independent auditors, noting "a critical process failure in [the WHO's] evidence-gathering and public health recommendations" about a wide variety of infectious illnesses, "seemingly stemming from poor citation practices and a serious misallocation of investigative resources."

Director-General Tedros pledged to provide daily updates on the quickly evolving state of the evidence, as researchers re-evaluated core assumptions about disease transmission under a deadline, mere days after the first confirmed case of the novel SARS-CoV-2 coronavirus outside China.

The WHO's briefing included their daily-updated probability distribution over infection fatality rates and reproductive rates from the...

  1. This article is a wild ride.
  2. They do not jest about the difficulty of acquiring the book (Airborne Contagion and Air Hygiene: An Ecological Study of Droplet Infections). It has no DOI number; Worldcat confirms it was digitized in 2009 but it must have been a weird method because it doesn't get referenced like other old books I've searched for. I did find at least one review that said the book was to airborne disease as the pumphandle investigation was to waterborne disease, which is about the highest conceivable endorsement. Put the damn thing back into print, Harvard!
  3. Katie Randall's historical research.
  4. Access to a PDF versions of a few articles co-authored by Linsey Marr:
    1. The indoors influenza article from 2011.
    2. Letter published in Science, Oct 2020.
    3. Minimizing indoor transmission of COVID, Sept 2020.
    4. A review in Science from Aug, 2021
  5. Almost everything by Firth and co is unavailable.
    1. A first page of Firth's tuberculosis rabbits experiment, 1948.
    2. The guinea pig and UV study, done by Firth's student Richard Riley, 1962.

I have examined none of these in depth, but the publications all appear to be real and also make the reported claims. However, I notice that when you start from Firth, information about this was pretty widespread in the 2010-2019 timeframe. We had plenty of time not to screw this one up.

I feel like agencies who make recommendations to the public, either as a matter of routine or in times of crisis, should have a historian of science on staff whose job is to discover and maintain the intellectual history of these recommendations. This way we will know how to update them in light of whatever current crisis.

A Cached Belief

I find this Wired article an important exploration of an enormous wrong cached belief in the medical establishment: namely that based on its size, Covid would be transmitted exclusively via droplets (which quickly fall to the ground), rather than aerosols (which hang in the air). This justified a bunch of extremely costly Covid policy decisions and recommendations: like the endless exhortations to disinfect everything and to wash hands all the time. Or the misguided attempt to protect people from Covid by closing public parks and playgrounds, which pushed people to socialize indoors instead.[1]

According to the medical canon, nearly all respiratory infections transmit through coughs or sneezes: Whenever a sick person hacks, bacteria and viruses spray out like bullets from a gun, quickly falling and sticking to any surface within a blast radius of 3 to 6 feet. If these droplets alight on a nose or mouth (or on a hand that then touches the face), they can cause an infection. Only a few diseases were thought to break this droplet rule. Measles and tuberculosis transmit a different way; they’re described as “airborne.” Those pathogens travel inside aerosols, microscopic particles that can stay suspended for hours and travel longer distances. They can spread when contagious people simply breathe.

The distinction between droplet and airborne transmission has enormous consequences. To combat droplets, a leading precaution is to wash hands frequently with soap and water. To fight infectious aerosols, the air itself is the enemy. In hospitals, that means expensive isolation wards and N95 masks for all medical staff.

Finally, here's a 2006 paper by Lidia Morawska, who features prominently in the article, on droplet transmission:

This paper reviews the state of knowledge regarding mechanisms of droplet spread and solutions available to minimize the spread and prevent infections.

Practical implications: Every day tens of millions of people worldwide suffer from viral infections of different severity at immense economic cost. There is, however, only minimal understanding of the dynamics of virus-laden aerosols, and so the ability to control and prevent virus spread is severely reduced, as was clearly demonstrated during the recent severe acute respiratory syndrome epidemic. This paper proposes the direction to significantly advance fundamental and applied knowledge of the pathways of viral infection spread in indoor atmospheric systems, through a comprehensive multidisciplinary approach and application of state-of-the-art scientific methods. Knowledge gained will have the potential to bring unprecedented economical gains worldwide by minimizing/reducing the spread of disease.

This potential proved harder to realize than expected.

On Trusting the Experts

This story is one of the lessons from the Covid years which I come back to most often. The screw-up informs how I think about questions of expertise, like to which extent I can trust experts and whether experiences from the Covid pandemic should reduce that trust.[2]

And what does it even mean to "trust the experts", when there are multiple factions which claim expertise on a topic?

From the article, about a Zoom meeting in April 3, 2020:

[The] new coronavirus looked as if it could hang in the air, infecting anyone who breathed in enough of it... But the WHO didn’t seem to have caught on. Just days before, the organization had tweeted “FACT: #COVID19 is NOT airborne.” That’s why ... [36 aerosol scientists were] trying to warn the WHO it was making a big mistake.

Over Zoom, they laid out the case. They ticked through a growing list of superspreading events in restaurants, call centers, cruise ships, and a choir rehearsal, instances where people got sick even when they were across the room from a contagious person. The incidents contradicted the WHO’s main safety guidelines of keeping 3 to 6 feet of distance between people and frequent handwashing. If SARS-CoV-2 traveled only in large droplets that immediately fell to the ground, as the WHO was saying, then wouldn’t the distancing and the handwashing have prevented such outbreaks? Infectious air was the more likely culprit, they argued. But the WHO’s experts appeared to be unmoved. If they were going to call Covid-19 airborne, they wanted more direct evidence—proof, which could take months to gather, that the virus was abundant in the air. Meanwhile, thousands of people were falling ill every day.

On the video call, tensions rose. At one point, Lidia Morawska, a revered atmospheric physicist who had arranged the meeting, tried to explain how far infectious particles of different sizes could potentially travel. One of the WHO experts abruptly cut her off, telling her she was wrong, Marr recalls. His rudeness shocked her...

Morawska had spent more than two decades advising a different branch of the WHO on the impacts of air pollution. When it came to flecks of soot and ash belched out by smokestacks and tailpipes, the organization readily accepted the physics she was describing—that particles of many sizes can hang aloft, travel far, and be inhaled. Now, though, the WHO’s advisers seemed to be saying those same laws didn’t apply to virus-laced respiratory particles. To them, the word airborne only applied to particles smaller than 5 microns. Trapped in their group-specific jargon, the two camps on Zoom literally couldn’t understand one another.

So the article obviously takes the side of the aerosol scientists, including calling one "revered", and calling a WHO expert "rude". And given that the WHO eventually relented on this issue[3], that makes sense. But an article which takes sides more than a year after the fact doesn't help us as much to decide which experts to trust in the moment.

That being said, when a big organisation like the WHO makes factual claims with far-reaching economic consequences, and is then very slow to change its mind, and to my knowledge neither apologizes for the mistakes nor fires or even just reprimands anyone responsible, that certainly makes me trust it a lot less.

Conversely, I studied physics, so I can just follow my own tribal instincts and decide to trust the physicists over the doctors.

Maybe it's easy to decide which experts to trust, after all!

  1. ^

    I wonder how a what-if scenario would've worked out where everything about Covid stayed the same, except that this cached belief had been corrected before 2020.

  2. ^

    Unfortunately I don't have any great answers here. Experiences like these mostly push me towards more skepticisim and epistemic learned helplessness.

  3. ^

    This article in Nature has a timeline of the slooowly evolving WHO statements:

    From 9 July 2020:

    ... short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out.

    From 20 October 2020:

    "Current evidence suggests that the main way the virus spreads is by respiratory droplets among people who are in close contact with each other. Aerosol transmission can occur in specific settings, particularly in indoor, crowded and inadequately ventilated spaces, where infected person(s) spend long periods of time with others, such as restaurants, choir practices, fitness classes, nightclubs, offices and/or places of worship."

    From 23 December 2021:

    “Current evidence suggests that the virus spreads mainly between people who are in close contact with each other, for example at a conversational distance … 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 can remain suspended in the air or travel farther than conversational distance (this is often called long-range aerosol or long-range airborne transmission).”

A few years after the fact: I suggested Airborne Contagion and Air Hygiene for Stripe’s (reprint program)[].