Where did the 5 micron number come from? Nowhere good. [Wired.com]

by Elizabeth1 min read9th Nov 20216 comments


Practice & Philosophy of ScienceMedicineHistoryScholarship & LearningCovid-19World Modeling

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.