Could WHO’s Technical Report on “Pathogens That Transmit Through the Air” Already Be Obsolete, Two Weeks After Its Release?

By Lambert Strether of Corrente.

Readers will recall that a mere fourteen days ago I reviewed WHO’s long-awaited report on aerosol tranmission. published April 18, 2024: “WHO’s New Technical Report on “Pathogens That Transmit Through The Air” (with a Note on the Pandemic Treaty)” (Report). My view was that the report was in fact more political than technical, and in fact represented a comprehensive win for the forces of light, i.e. aerosol scientists:

While many #CovidIsAirBorne advocates (including this humble blogger) are not completely happy with some of the language, I believe that the Report represents a comprehensive technical defeat for the “droplet dogmatists” (although an incomplete institutional defeat, which sadly must take place “one funeral at a time,” though hopefully with dispatch).

Incomplete though the defeat of the droplet dogmatists was — they were not, sadly, loaded with chains, taken outside the city walls, and stoned — it was nevertheless a defeat; aerosol scientists would henceforth have the seat at the table hitherto denied them. However, the Report seemed technically accurate as far it went, battles over semantics and naming conventions aside.

However, I recently discovered this article, from February 2024, in Fluids (not the sort of journal widely read Infection Control): “Airborne Transmission of SARS-CoV-2: The Contrast between Indoors and Outdoors” (Indoors and Outdoors), an “analytical review (which also includes original experimental and analytical work).” Caveats: I’m not an aerosol scientist, obviously. Nor would I normally devote a post like this to a single article, and MDPI, though peer-reviewed, is not top-drawer; Fluids has an impact factor of 1.9, where 10.0 is considered excellent and 3.0 is considered good. Speaking in the article’s favor, it’s part of a special issue titled “Computational Modelling of Particle Flows in Environmental and Bio-Transport Applications.” The authors are all from peripheral institutions, like Leeds Beckett University (though peripheral is not the same as bad; think MMT, blackballed from the Ivies) and have done relevant work previously, although none of them were participants in the Report. Plus there are plenty of equations and models. In any case, for Covid, indoor transmission is supremely important, and for that reason alone Indoors and Outdoors deserves a look.

However, I’m really persuaded by Indoors and Outdoors’s narrative of how air moves; having thought a lot about that when I weather-proofed and insulated my house, I find their narrative instantly obvious and intuitive; you could summarize it as “hot air rises.” I think you will be persuaded by that narrative as well, and if you are, that means, as I urged, that the Report is already in need of revision (and there is a major error in the science as well, which we’ll get to). So, first I will extract what the Report has to say about tranmission; then I will go through Indoors and Outdoors, and display its narrative.

Tired: Short and Long Distances

A simple binary distinction between short and long distance transmission of Infectious Respiratory Particles (IRPs) “through the air” pervades the Report. Going page by page:

Page xii:

i) Airborne transmission/inhalation: Occurs when IRPs expelled into the air (as described above) and enter, through inhalation, the respiratory tract of another person. This form of transmission can occur when the IRPs have travelled either short or long distances from the infectious person (28, 37, 41, 43, 53, 63, 84, 91–96)

Page 10:

The phrase ‘transmission through the air’ can be used to describe the transmission of IRPs through the air, via either airborne transmission/inhalation or direct deposition modes (or other labels matching equivalent descriptions) as outlined above. This can therefore include the transmission of IRPs on a spectrum of sizes, over both short and long distances.

Page 11:

This can therefore include the transmission of IRPs on a spectrum of sizes, over both short and long distances. See Figure 1 and Table 1 for schematic descriptions of these modes of transmission (and other related transmission modes for completeness)

Page 12:

There is NO suggestion from this consultative process that to mitigate the risk of shortrange airborne transmission full ‘airborne precautions’ (as they are currently known) should be used in all settings, for all pathogens, and by persons with any infection and disease risk levels where this mode of transmission is known or suspected (126)

Page 15:

Reaching consensus on the term ‘infectious respiratory particles’, moving away from a strict dichotomy of particle sizes, and accepting that smaller IRPs can be transmitted at both short and long-range depending on several influencing factors, are all major achievements.

Pages 32-33:

If, as is recognized herewith, smaller IRPs are capable of being transmitted at both short and long-range, then to effectively counteract this risk, full (what is now known as) ‘airborne precautions’, which involves substantive IPC measures, such as use of respirators, with or without specialized hospital rooms etc., may need to be applied to all those at risk of the disease, if a precautionary principle is to be applied or applied selectively depending on the frequency, morbidity, and treatment options for different pathogens (which may vary widely between and within countries).

Now, to be fair, the Report includes the text “There are many factors that can influence the particle distribution, spread and subsequent effect on an individual of exhaled IRPs,” after which follows a long bulleted list including host, Pathogen characteristic, particle size, environmental conditions, etc. However, over and over again, the Report throws transmission “through the air” into two buckets: Short range, and long range. I am persuaded by Indoors and Outdoors’s narrative that this binary is not science-based (though in the Conclusion I will speculate on what it is based). Now let us turn to Indoors and Outdoors.

Wired: Near- and Far-Field Transmission, with Feedback Loops

Indoors and Outdoors includes a lot of material that is above my paygrade: the “Wells–Riley model,” “Schlieren visualisation and computational fluid dynamics (CFD) based on the Eulerian–Lagrangian approach.” (No doubt we have readers who are expert in this material, and I hope they will commment.

Now let’s turn to a series of extracts from Indoors and Outdoors. From the Abstract (mapping “short range” to “near field,” and “long range” to “far field”):

COVID-19 is an airborne disease, with the vast majority of infections occurring indoors. In comparison, little transmission occurs outdoors. Here, we investigate the airborne transmission pathways that differentiate the indoors from outdoors and conclude that profound differences exist, which help to explain why SARS-CoV-2 transmission is much more prevalent indoors. Near- and far-field transmission pathways are discussed along with factors that affect infection risk, with aerosol concentration, air entrainment, thermal plumes, and occupancy duration all identified as being influential…. Pathways of airborne infection are discussed, with the key differences identified between indoors and outdoors. In particular, the contribution of thermal and exhalation plumes is evaluated, and the presence of a near-field/far-field feedback loop is postulated, which is absent outdoors.

The key point here is that if there is a “near-field/far-field feedback loop” then the WHO binary is wrong; WHO has no concept that short- and long-range transmission interact (and to be fair to WHO, the idea never occurred to me, either).

Some hitherto ignored issues:

[R]elatively little work has focused on the differences that exist between the two environments [indoors and outdoors] regarding fluid dynamics and the behaviour of respiratory aerosols, with the result that some important issues have largely been overlooked. More specifically, little attention has been paid to issues such as the interaction between thermal plumes and ceilings [“hot air rises”]; the age of the inhaled aerosols; the impact of poor air mixing; and the contribution that far-field airborne viral load makes to near-field exposure—all issues that appear to be influential in the transmission of SARS-CoV-2 indoors.

The paper includes material on assessing indoor infection risk (the Wells–Riley model), but I’m not equipped to evaluate it, so I will skip ahead to aerosol transmission. From Section 3:

In rooms and other enclosed spaces, the infectious aerosol particles mentioned above pose both a ‘near-field’ and a ‘far-field’ threat, with the near-field being close proximity to the infector (i.e., <2 m) and the far-field generally considered >2 m away. The near-field transmission risk occurs due to the cone-shaped cloud of aerosol particles [WHO’s “puff cloud”] that are exhaled when speaking, singing, shouting, or breathing and which has the potential to infect susceptible individuals in close proximity. This aerosol cloud is turbulent and expands in volume as it entrains air from the surrounding room space… Importantly, near-field transmission has a directional component, with face-to-face interactions generally posing a greater risk compared with side-by-side or back-to-back spatial arrangements. By comparison, the far-field transmission risk arises when the aerosol particles have been dispersed by air currents into the wider room space. It is termed ‘far-field’ because the dispersed aerosols pose a threat to all those who are in the same space but not in close vicinity to an infector.

From Section 6:

Another fundamental difference between internal and external environments is that indoor spaces have ceilings, and outdoor spaces generally do not. In terms of fluid dynamics, this simple and often ignored difference has a profound effect on aerosol transport in the two environments. All human beings are surrounded by a personal thermal plume comprising upwards flowing convective air currents [“hot air rises”]…. When they reach the top of the room, the aerosols tend to fan out along the underside of the ceiling due to the convection current and travel horizontally some distance before descending back towards the floor and travelling through the breathing zone. This interaction between thermal plumes and ceilings is unique to the internal environment and is one of the main drivers of air circulation within room spaces.

And:

One of the characteristics of human thermal plumes is that the upwards convection currents associated with them start at the floor and travel along the legs. This can cause horizontal air currents to occur at floor level in room spaces, which are capable of transporting the smallest respiratory aerosols without them settling on the floor. So, while larger aerosol particles will tend to settle out on the floor due to gravitational deposition, the finest aerosols will tend to remain airborne. Furthermore, when these fine aerosols reach the thermal plume of another human being, they will become entrained into that plume and travel upwards through the breathing zone of that individual. While the extent to which this phenomenon contributes to COVID-19 transmission indoors is unknown, it is completely absent outdoors and is therefore worthy of further investigation.

From Section 7:

While many analysis techniques assume that the air in room spaces is well mixed, this is often not the case, especially in poorly ventilated spaces. In particular, the action of thermal and exhalation plumes can cause regions of high aerosol concentration to form… High aerosol concentrations can also occur in stagnant regions that are poorly ventilated, especially if eddy currents are present that cause particles to become trapped in a specific location. Furthermore, pressure gradients can cause concentrations to increase downstream of a source as more and more aerosol particles are introduced into the air stream. Collectively, this means that occupants may experience different levels of SARS-CoV-2 exposure while located in the same room space due to incomplete air mixing, allowing regions of high and low aerosol concentration to coexist at the same time…. [T]reating rooms as if they are completely mixed is likely to lead to an underestimation of the infection risk posed to some individuals.

From Section 8:

While a full discussion of weather-related issues is beyond the scope of this paper, the extent to which the climate influences the fluid transport of aerosols in buildings and other enclosed spaces is relevant. For example, in northern Europe, where the climate is cold or cool for much of the year, people tend to spend much time indoors in buildings with windows shut for comfort and energy-saving reasons. In such circumstances, ventilation rates will tend to be low, generally causing the concentration of respiratory aerosols in room air to increase. By contrast, in hotter countries, people may be more willing to open windows to promote ventilation for comfort reasons. Also, the general behaviour of building occupants may differ from that exhibited in more northerly countries, with individuals spending more time outdoors. In addition, the use of ceiling fans and air conditioning units in warmer countries will strongly influence the fluid dynamics of the air in buildings.

And the feedback loop:

Two issues in particular, the interaction between thermal plumes and ceilings and the entrainment of room air into exhalation plumes, appear to have been largely overlooked in the literature…. Yet indoors, they present a major challenge because rooms are by definition confined and generally have ceilings. This means that indoors: (i) boluses of respiratory aerosols (high-concentration clouds) will tend to form at the ceiling and travel horizontally before descending through the breathing zone of the room occupants; and (ii) as the concentration of respiratory aerosols builds up in the room space, so the near-field exposure risk associated with exhalation plumes will tend to increase. Both these phenomena mean that the risk of transmission is much greater indoors compared with outdoors. They also highlight the inadequacy of the simplistic ‘near-field’–‘far-field’ analysis framework. In reality, in most indoor environments, the near-field and far-field exposure risks are inextricably linked—something that is not the case outdoors. Because exhalation plumes entrain air from the surrounding room space, a feedback loop exists between the far-field and near-field. Similarly, because thermal and exhalation plumes are major drivers of circulation within room spaces, they cause respiratory aerosols to become widely dispersed, with the result that indoors, near-field risks can be projected considerable distances into the far-field. Consequently, profound differences in respiratory aerosol behaviour exist between the indoor and outdoor environments, with no clear boundary separating the near and far-fields indoors.

(Note that the concept of “boluses of respiratory aerosols (high-concentration clouds)” is not present in WHO’s Report. From page xii:

“IRPs exist in a wide range of sizes (from sub-microns to millimetres in diameter). The emitted IRPs are exhaled as a puff cloud (travelling first independently from air currents and then dispersed and diluted further by background air movement in the room).”

But WHO is wrong. IRPs are not diluted; they are concentrated near the ceiling (“hot air rises”).

And:

In particular, we have identified that a feedback loop exists between the near-field and the far-field inside buildings, which is completely absent outdoors. This feedback loop is facilitated by the action of the exhalation and thermal plumes associated with occupants in room spaces. These plumes drive much of the air circulation within rooms and can rapidly disperse respiratory aerosol particles throughout a space[1]. Although the dynamics of these plumes are complex and not fully understood, it appears that they play a key role in driving the spread of airborne diseases like COVID-19 and tuberculosis (TB) indoors

Once again, WHO is wrong; “short range” and “long range” are not separate buckets, in reality or as a classification device; there is a feedback loop between them, so they interact. They are not separate at all.

Conclusion

WHO’s terminology — a unified terminology being the ostensible purpose of the Report — is at best incomplete. We have, apparently, now standardized on transmission “through the air” and “puff clouds.” However, we have no terminology for “boluses of respiratory aerosols (high-concentration clouds)”, and “whereof one cannot speak, thereof one must be silent.” WHO’s science is bad too: IRPs are not diluted as they move away from the source. They are concentrated!

* * *

WHO’s Report is clearly the end product of an enormous institutional tussle (a tussle the droplet dogmatists lost). That struggle has been going on, quite visibly, for some time. But the persistance of an absolute “short range”/”long range” distinction is harder to explain (though, again, once you focus on the concept that “hot air rises” — hard to see how WHO missed that _{MR SUBLIMINAL Joke, ha ha!} the static distinction vanishes, to be replaced by a feedback loop).

Speculatingly freely on a future institutional tussle: Hospitals are enormously influential in the health care system, globally. We know we’ll have to pry Baggy Blues from the cold, dead hands of Infection Control; and it seems likely that hospitals will massively resist taking a unified approach to ventilation in their facilities (because of expense, because of liability, and because many slots in the administrative hierarchy depend on evading a unified approach; operating theatres, negative and positive pressure rooms, hallways, back offices, etc., are all treated as separate, although “Covid passes everywhere,” as Gurney Halleck did not quite say[2]).

One might imagine, then, that the utility of the “short range” and “long range” distinction has nothing to do with the science, and everything to do with the institutional tussle. Infection Control can argue that Baggy Blues control the short range (they don’t), and that the “long range” isn’t important (it is); hence the importance of that word “diluted.” We would thus end up, operationally, with droplet dogma, just without the droplets, and without the dogma. It will be very interesting to see if WHO’s Report appears in HICPAC’s next work product.

NOTES

[1] I would like very much to know what happens to the “bolus” within a room — e.g., a normal (not positive pressure) hospital room — when a door is opened, but Indoors and Outdoors does not say.) Apparently, opening a hinged door creates a vortex, but I would like that incorporated into the feedback model described here.

[2] It also seems likely that “IRP-conscious” health care workers left the profession when it became evident that hospitals were recklessly infecting both helpless patients and themselves, leaving the field to the “Let me see your smile” Nazis.

APPENDIX WHO’s Undying Shame

This tweet is still up:

FACT: #COVID19 is NOT airborne.

The #coronavirus is mainly transmitted through droplets generated when an infected person coughs, sneezes or speaks.

To protect yourself:
-keep 1m distance from others
-disinfect surfaces frequently
-wash/rub your 👐
-avoid touching your 👀👃👄 pic.twitter.com/fpkcpHAJx7

— World Health Organization (WHO) (@WHO) March 28, 2020

We know for a fact that WHO will issue erratum tweets to correct errors. So it’s hard to give an account for this tweet’s continued existence. One might almost conclude that the tweet under some form of protection, perhaps from some influential functionary, or possibly even a fundraiser. (Personally, nuking this abomination of a tweet would be my first requirement before even considering a role for WHO in the pandemic treaty. I mean, how is it possible for an institution that wrote the Report to leave this tweet up?)