The World Health Organisation.  At the WHO's first international press conference on the new, as-yet unnamed coronavirus on 11 th February 2020 (source A2, Table 1), its Director-General began by emphasising the importance of handwashing and using paper tissues to catch sneezes. Whilst he went on to state (page 10) that "corona[virus 19] is airborne", he shortly afterwards corrected himself:
 "Okay. Sorry, I used the military word, airborne. It meant to spread via droplets or respiratory transmission. Please take it that way; not the military language. Thank you." (source A2, Table 1, page 12)
As its current guidance on the topic (source A1, Table 1   ) states, the term 'military' reflects the WHO's advisory role on biological weapons, in which unknown respiratory pathogens are routinely classified as airborne threats:hence, potentially extremely perilous:until firm evidence allows them to be reclassified.
The corrected message that SARS-CoV-2 was "not airborne" had limited impact initially - public health officials inspecting outbreaks in newly-affected countries donned scarce hazmat suits, for example. In the face of what appeared to be excessive caution by these countries, and in the context of a global shortage of PPE   , the WHO found it necessary to underline its message with a 'fact check' Tweet (source A5, Table 1) on the 28 th March 2020 stating "COVID-19 is NOT airborne" to counter what it called 'fake news' about potential airborne spread and re-emphasise contact and droplet modes of transmission.
This uncompromising message from the WHO in March 2020 may partly explain why politicians and policymakers in the West rapidly aligned with a droplet theory of transmission and ignored, downplayed, or rejected the work of scientists proposing an airborne route.
Scientific briefings produced by the WHO (e.g., source A11, Table 1) and national policy bodies (see next sections) reveal longstanding errors in the science of bioaerosols which have been perpetrated in derivative publications over the years. In particular, three flawed assumptions, whose origins can be traced to the limitations of scientific equipment in the late 19 th and early 20 th centuries   , distorted the thinking of policymakers and the non-expert scientists who were advising them: that there is a clear dichotomy between droplets (above five microns) and aerosols (below five microns); that the former explain transmission of respiratory diseases within two metres whereas the latter account only for transmission beyond that distance; and that transmission dynamics of bioaerosols in coughs, sneezes and exhaled air are linear and predictable  ,  . These three oversimplifications may have been unconsciously seized upon by policymakers through a process of satisficing - that is, in the face of urgency, threat, and uncertainty, ensuring that their decisions made sense and were accountable within a selected range of parameters   . Droplet theory, especially when anything below five microns was incorrectly defined as a droplet (in reality, particles of up to 100 microns can be carried long distances in the air), made possible the individualist "how to protect yourself..." message (source A4, Table 1) based on personal cleanliness and a simple physical distancing rule. If these were the key measures needed, the WHO would not have to concern itself with such matters as the chemistry of air composition, the physics of air flow, or the architecture of the built environment; it reduced the risk of mass panic at the idea of uncontrolled spread of a new and poorly-understood disease through the very air we breathe; and it made the worldwide shortage of PPE less urgent.
The WHO's position on prevention of COVID-19 up to early 2021 was based largely on advice from its Infection Prevention and Control Research and Development Expert Group for COVID-19 (IPCRDEG-C19). Most members of that committee are hospital clinicians with specialist training in infectious diseases; they were also strong adherents to the tenets of evidence-based medicine, which is based on empiricist assumptions and reluctant to consider types of evidence beyond randomised controlled trials, as its briefing document on guideline development attests (source A20, Table 1). As public health professor Raina MacIntyre described in a blog (source A15, Table 1), these committee members are experts in topics such as wound management:for which droplet spread is predominant and handwashing is an effective intervention. A leading member of the IPCRDEG's Secretariat recently led an international handwashing campaign "to consistently improve hand hygiene practices as a whole-of-society approach to stop the spread of SARS-CoV-2" (source A18, Table 1   ).
On 6 th July 2020, 238 aerosol scientists from around the world published an open letter addressed to international policymaking bodies:among which the WHO was implicitly highlighted:summarising studies undertaken by its signatories which had demonstrated "beyond any reasonable doubt" that the SARS-CoV-2 virus is released in particles small enough to be carried long distances in the air when people talk, cough, and even just exhale (source A8, Table 1   ). Three days later, following press coverage of the letter:some somewhat negative (source A9, Table 1   ) and some more positive (source A10, Table 1   ):the WHO published a new Scientific Brief on Transmission of SARS-CoV-2 (source A11, Table 1   ). This document, which remains current at the time of writing, acknowledged the existence of various aerosol studies but considered those studies flawed in various ways and concluded that "transmission of SARS-CoV-2 by this type of aerosol route [i.e. coughing, speaking, singing, breathing] has not been demonstrated"   . 
A few weeks later, members of the IPCRDEG-C19 committee published an article (source A12, Table 1   ) with its Chair as lead author, declaring that "Multiple clinical and epidemiologic reports have now lent considerable support that the predominant route of human-to-human transmission of the SARS-CoV-2 is through respiratory droplets and/or contact routes and do not support significant airborne transmission" (7, page 2). The academic sources cited in that paper to substantiate the droplet theory were remarkably sparse: they consisted of a report of person-to-person transmission within a single family   , a single hospital case in which air samples had been negative for the virus   , and a single air flight in which nobody got infected   . A letter to the editor (source A14, Table 1   ) argued that the paper was highly selective in its citation of evidence (e.g., it omitted several studies which had found viable virus in the air) and included some fundamental errors of reasoning (e.g., conflating what the authors took to be lack of evidence in favour of aerosol spread with evidence refuting such spread)   .
Some WHO documents contain a key logical fallacy that dominance of close-contact transmission excludes a major role for aerosols. For example, in its July scientific brief (source A11, Table 1), the WHO states: "Current evidence suggests that transmission of SARS-CoV-2 occurs primarily between people through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions, or through their respiratory droplets"   . This flawed logic was widely reproduced in national-level scientific briefings. For example, the US Centers for Disease Control and Prevention's scientific brief from October 2020 states: "The epidemiology of SARS-CoV-2 indicates that most infections are spread through close contact, not airborne transmission"   . As noted above, aerosol transmission occurs predominantly at close contact   , so dominance of close-contact transmission cannot be taken as evidence that droplet mode dominates.
Whilst the WHO has shifted its position substantially since the beginning of 2021 (for example, recommending ventilation of indoor spaces alongside handwashing and physical distancing from March 2021 - source A17, Table 1   ), its guidance at the time of writing remains primarily focused on measures to reduce close-range droplet transmission except in the specific circumstances of AGMPs, as its living guideline (source A16, Table 1   ) points out. A living systematic review of the role of airborne transmission commissioned by the WHO and including the Chair of the IPCRDEG-C19 as a co-author was published as a preprint on 24 th March 2021 (source A19, Table 1   ). It considered that no firm conclusions could be drawn about airborne transmission and observed that "Among case clusters for which airborne transmission is hypothesised, published detailed investigations cannot rule out that droplet and fomite transmission could also explain human-to-human transmission." (6, page 4). Notably, neither the living guideline nor the living systematic review included any aerosol scientists as co-authors. 
The members of the WHO's IPCRDEG had impressive credentials in the fields of both medical science and national policymaking. Its Chair, for example, is a Professor of Medicine and past President of the Canadian Infectious Disease Society, past Board Chairman of the Canadian Committee on Antibiotic Resistance, and a recipient of a Distinguished Service Medal from the Alberta Medical Association and the Order of Canada for Services to Medicine (source A21, Table 1). A Bourdieusian analytic lens would observe that the higher an individual's endowment of capitals, the stronger the stakes in a game they would have. In such a high-stakes game, highly endowed become the custodians of power, privilege, and boundary conditions of the game. Given their position of seniority and legitimacy within the field and the associated capitals, it would be difficult for them to challenge the rules of the game (the illusio) from within and to accept knowledge from the margins.
  United Kingdom.  Our UK example centres around an open letter sent to the UK Prime Minister on 19 th February 2021, led by the Royal College of Nursing and signed by 18 other healthcare workers' organisations including paramedics, podiatrists, nutritionists, speech and language therapists, and porters (source B3, Table 1   ). The authors asked for better ventilation and higher-grade PPE across a wide range of healthcare settings and extending beyond AGMPs.
The open letter challenged the UK's current COVID-19 Infection Prevention and Control (IPC) Guidance (source B1, Table 1   ), updated in January 2021, which restricted higher-grade PPE to staff:mostly senior doctors:performing AGMPs. It also criticised a state-commissioned 'rapid review' document published by the Antimicrobial Resistance and Healthcare Associated Infection (ARHAI) (source B2, Table 1   ), on which the IPC guidance was based. The ARHAI review was labelled 'Version 11.0'; it concluded (as the previous 10 versions had done) that the predominant mode of transmission was large droplets at short range and that there was " no clear evidence" (page 9) for airborne spread outside AGMPs; a version 12 published a few weeks later contains the same claims.
An independent evidence assessment commissioned by the Royal College of Nursing (source B4, Table 1   ) described the ARHAI rapid review as flawed and outdated. Front-line nurses gave media interviews describing their concerns about working with minimal protection when patients were unwell and coughing, backed up by aerosol scientists who agreed that coughing would generate virus-laden aerosols for which standard masks were inadequate protection. In one such interview, the presenter reminded the audience that Health Protection Scotland's latest advice is that there is no evidence to support a change in recommendations and that the Health Secretary had said they would be ''guided by the experts' (a reference to infectious diseases doctors). She is quoted:
 We take that really seriously and have adapted the PPE that we provide. But we also have a situation where individual members of staff in NHS or in care are able to exercise their own professional judgement about the circumstance that they face and whether or not they believe that they need additional PPE to that that is currently clinically recommended (Scottish Health Secretary, quoted on Good Morning Scotland, 2 nd March 2021 - source B5, Table 1)
We consider this deflection to 'own professional judgement' further in our analysis section.
Also noteworthy in this case is the publication on 5 th March 2021 of new Public Health England guidance on ventilation of indoor spaces (source B6, Table 1   ). In an exact mirror of the WHO:and citing the WHO's new roadmap on ventilation, published a few days earlier (source A17, Table 1   ):this new policy document appeared to base its recommendations on an assumed airborne route of transmission but was not accompanied by substantial changes to its other policy documents.
A key player in this case study is the instigator and lead author of the letter to the Prime Minister, the Professional Lead of the Infection Prevention and Control Network at the Royal College of Nursing (RCN) (source A22, Table 1). She is a registered nurse, with many years' clinical experience along with a two-year secondment managing professional standards at the RCN. The Professional Lead role requires her to engage with front-line nurses and ensure the highest standards of infection control in their clinical work. She is the person within the nurses' professional body to whom registered nurses would direct their concerns about unsafe practices. As the letter she drafted (source B3, Table 1   ) illustrates, she appears to feel a strong sense of moral responsibility to prevent further deaths of frontline healthcare workers.
  Canada.  In this case study, we focus on guidance relating to schools in one Canadian province, British Columbia, and a legal challenge by healthcare workers in another province, Quebec.
From the very beginning of the pandemic, British Columbia based its prevention measures on an explicit contact, droplet, and fomite theory of transmission. A tweet posted by from British Columbia's Centre for Disease Control (source C1, Table 1) on 11 th February 2020, for example, linked to a video by a physician epidemiologist and stated: "The new #coronavirus is spread by droplets that come from the mouth or nose. The droplets don't stay floating in the air. This is not an airborne virus." 
Many interventions, such as the state-wide policy of closing children's playgrounds and disabling traffic light push buttons, revealed a prevailing fear of droplet and fomite transmission. But as evidence on airborne transmission accumulated during the late spring and summer of 2020, pressure from workers' unions, supported by aerosol scientist researchers, mounted for the province to adopt measures to reduce airborne spread.
However, the province authorities long resisted the idea of airborne transmission. British Columbia's Provincial Health Officer described the open letter from 238 international aerosol scientists as " a little bit of a tempest in a teapot [...]" and reiterated her confidence in the existing advice focused on large droplets and surface transmission (source C3, Table 1   ). It was not until early January 2021 that the British Columbia Center for Disease Control (BCCDC) edited its guidelines to include the risk posed by "smaller droplets" (which may be a euphemism for aerosols):
" Smaller droplets come out of the mouth and nose at the same time as larger droplets. These smaller droplets are light, and they can float in the air for a longer time. Because of this, smaller droplets may collect in enclosed spaces unless they are diluted with clean air from the outdoors or from a ventilation system. If many people are sharing a space without enough clean air, it can lead to COVID-19 infections." (source C6, Table 1   )
Despite stopping short of the use of words such as 'aerosols' or 'airborne', the new guideline was welcomed by front-line workers. But it still provided ambiguous messages on masks and omitted any directives to improve ventilation in schools. Extraordinarily, school officials in one locality ordered classroom windows to be screwed shut after teachers had opened them to increase ventilation (source C11, Table 1   ).
The Provincial Health Officer for British Columbia is a highly-respected medical doctor. She had initial specialist training in infectious diseases and a distinguished career in military medicine, as well as working for the WHO internationally, and subsequently trained in public health. She was the public health lead in Toronto for the 2003 SARS outbreak, and in 2018 was the first woman to be named as chief medical officer for British Columbia. Her regional role during the Covid-19 pandemic, including regular media briefings to explain aspects of the science to the public, made her a household name in the state and conferred legitimacy and authority on her statements (source C12, Table 1). She appears at least partly driven by the urge to quell panic and maintain calm:which may explain her 'storm in a teacup' comment and, more generally, her reluctance to embrace theories about the dominance of aerosol transmission.
In Quebec, a dispute about airborne transmission of the virus ended up in the courts. In April 2020, during the first wave of the pandemic, Quebec's Nurses Union (FIQ) asked the province's National Institute of Public Health (INSPQ) that they mandate the use of N95 masks (a kind of respirator providing high-grade protection against aerosols) in long term care facilities where most of the COVID-19 cases were occurring (source C13, Table 1). However, INPSQ's advice was based on a theory of large droplet and fomite transmission (source C16, Table 1) and this view shaped the institutional response. It was illustrated, for example, by a report from around the same time in which one regional hospital network publicly blamed employees' sloppy handwashing to explain a COVID-19 outbreak (source C14, Table 1).
The first wave of the Covid19 pandemic took a very hard toll on Quebec's long-term care residents and workers. Between March 2020 and February 2021, 13% of the 40,000 people institutionalized in a long-term care institution in Quebec died from Covid-19 (source C22, Table 1). In some institutions, virtually all residents became infected and so few workers remained that at one point the Canadian military was called in to take over (source C15, Table 1). Notably, the army provided N95 masks for all their troops undertaking this work (source C15, Table 1).
Nevertheless, aligning with the orthodox view, Quebec's Director of Public Health published an ordonnance on 8 th June 2020 forbidding the use of N95 masks for health professionals save for a few designated procedures (source C17, Table 1). On 10 th July 2020, after losing hope of finding a negotiated settlement on their request for N95 masks, the FIQ brought the matter to a labour tribunal (source C18, Table 1). The Nurses Union claimed there was no shortage of N95 masks and that denying nurses this higher-grade protection was going to cost lives (source C19, Table 1). The availability of sufficient N95 masks was later confirmed by the minister of health (source C20, Table 1). By that time, more than 17,000 health care workers had been infected (source C20, Table 1).
The legal action ended with a scathing legal ruling in March 2021. The employer had argued, through an expert witness from the Public Health Institute of Quebec (INSPQ), that there was no evidence of significant airborne transmission of SARS-CoV-2 and that standard medical masks were adequate protection. In upholding the nurses' case, the judge decried INSPQ's expert for using 'false arguments' and 'fallacies ', and another expert for being unaware of key scientific developments in his field when testifying . The ruling also noted that, "despite [an] emerging scientific consensus, the INSPQ maintains, in court, the position that there is no air transmission" (source C22, Table 1 paragraph 143). Interestingly, shortly afterward, a group of 109 health professionals, most of them doctors and nurses from infectious diseases backgrounds, wrote a public letter contesting the scientific soundness of the ruling (source C23, Table 1). The letter states, for example:
 "Given the lack of scientific evidence regarding the benefits of wearing a respirator over a procedural mask when in contact with 'medium-risk' patients, we suspect that your recommendations are likely heavily influenced by the 'precautionary principle'. However, the precautionary principle must be based on science and not obscure it. In addition, this precautionary principle could prove to be deleterious for all workers in Quebec if the risks associated with your recommendations are greater than the expected benefits. We therefore ask that the CNESST [Commission des Normes, de l'Equite, de la Sante et de la Securite du Travail (French: Committee on Standards, Equity, Health and Safety at Work; Canada)] 
 carry out as soon as possible and publish a risk-benefit analysis of its recommendations. This analysis should include a forecast of the expected benefits including a quantification of the cases of infection prevented through these recommendations, as well as the number of RPAs that should be used to prevent infection in a worker (known as "Number needed to treat"). In addition, this analysis must include an estimate of the occupational injuries that will occur as a result of these recommendations. Prolonged wearing of the respirator is indeed associated with risks for workers (skin lesions, headaches, fatigue and increased risk of becoming infected, etc.)." (source C23, Table 1)
The phrasing of the letter strongly emphasises the potential harms of the N95 mask (including, allegedly, a worker becoming so fatigued that they become inadvertently infected) rather than its potential benefits (preventing infection and death). The extract also illustrates the authors' appeal to the tools of evidence-based medicine (such as the Number Needed to Treat metric   ) and their irritation in the face of what they appear to perceive as a challenge to their legitimate scientific authority in the sub-field of infectious diseases.
  Japan.  In Japan, cases and deaths from Covid-19 were very low compared to the West but high compared to neighbouring countries like Taiwan   . The first documented case of Covid-19 in Japan was on 15 th January 2020. By mid-February, there was an active national containment strategy in place which included assiduous contact-tracing with the goal of identifying, investigating, and quashing new clusters promptly   . Contact-tracing was both prospective (to identify onward transmission) and - unusually - retrospective (to identify which past activities may have led to the person becoming infected). Japan's approach also included lengthy quarantine periods (30 days), strict border controls, voluntary restriction of activities (for example, reducing travel and eating out, and working from home if possible), masking in the workplace, and economic support for businesses.
In contrast with the confident announcements from WHO and Western governments that the virus was not airborne, the Japanese government did not make any firm statements about the mode of transmission in these early documents. Notably, however, on 9 th March 2020, the Government of Japan (source D1, Table 1   ), with the personal backing of the Prime Minister, reported a careful analysis of several clusters across the country made possible through meticulous analysis of retrospective contact-tracing data. It introduced the '3Cs' message (avoid closed spaces, crowded places, and close proximity), explicitly invoking the precautionary principle on the grounds that the virus could be airborne:
 6. What we ask of you 
 The locations where mass infections were confirmed so far are places where the following three conditions were met simultaneously: (1) closed space with poor ventilation, (2) crowded with many people and (3) conversations and vocalization in close proximity (within arm's reach of one another). It is believed that more people were infected in such places. Therefore, we ask that you predict locations and settings where these three conditions could occur simultaneously and avoid them.
 We do not have enough scientific evidence yet on how significantly such actions can reduce the risk of spreading infection. However, since places with poor ventilation and crowded places are increasing infections, we ask that you take precautions even before scientific evidence for clear standards is found." (source D1, Table 1   , page 2)
Central to Japan's novel and successful strategy of retrospective contact tracing was a large cohort of highly-trained public health officers based in (or rapidly redeployed to) local public health centres - an approach sometimes referred to as field epidemiology. Importantly, these officers had ongoing experience of dealing with other airborne infectious diseases in the community, notably tuberculosis (TB), for which aggressive retrospective contact tracing with a view to identifying and controlling clusters was already in place   .
Japan's 3Cs message, which was designed to inform both national policy and public behaviour, was adopted into public-facing WHO advice on 13 th October 2020 (source A13, Table 1   ), though the mismatch between this advice and continuing statements elsewhere there was "no firm evidence" for airborne spread    was not addressed. Retrospective contact tracing had been attempted in some Western settings (e.g., British Columbia) in the first wave of the pandemic but was not sustained or showcased in the same way as Japan, nor did analysis of case clusters persuade local public health officials of the airborne nature of the disease. 
The striking difference in how aerosol theory was received in Japan compared to our other case studies may be explained in part by an analysis of individual-level factors. For example, the personality and trajectory of the virologist who is credited with introducing Japan's '3Cs' approach to the pandemic back in March 2020 (source D6, Table 1   ) - which structured his own habitus - likely played a role. He is a medical doctor by training and spent many years working as a senior adviser for the WHO on communicable diseases. He led WHO South East Asia's response to SARS in 2003, where he became known for promoting transparency and information-sharing. But this doctor took a novel approach to the COVID-19 pandemic. He worked with a professor of mathematics to analyse early clusters of cases both within Japan and on the Diamond Princess cruise ship. The discovery that:unlike with influenza:a tiny proportion of primary cases gave rise to around 80% of secondary cases led them to hypothesise that the airborne route of transmission was dominant and that the key to controlling the pandemic was controlling clusters   . A press article from November 2020 describes him thus:
 "X---, an unassuming and bespectacled 61-year-old, is at times hardly distinguishable from the average salaryman. A field epidemiologist by training, X--- cut his teeth working for Japan's development agency in Zambia, and has spent most of his career as an academic, currently affiliated with Tohoku University. He's far less well-known in his native country than other top infectious disease officials like Anthony Fauci in the U.S., and unlike Sweden's Anders Tegnell, no one is tattooing his image on their bodies. But those who worked with X--- say his early sense of urgency, constantly badgering government officials to do more, was crucial to Japan's response." (source D6   )
In contrast to many key public health actors in the West, then, this leading medical adviser in Japan kept a low public profile, decided that randomised controlled trial evidence and the assumptions of evidence-based medicine had little to contribute to the pressing questions around transmission, worked quickly and collaboratively with non-medical experts to produce a novel mathematically-informed hypothesis, embraced the precautionary principle (that action should be taken before scientific evidence is definitive), ignored advice from the WHO, developed a simple and catchy '3Cs' message, and operated quietly but effectively behind the scenes to bring political actors up to and including the Prime Minister on board.
This contrasts with his contemporaries in the West, whose vested interests in their own game and their decision to reject the contribution of heterodox scientists tied them firmly to contingent paths. The Japanese adviser was able to enjoy interdisciplinary collaboration in a country which seems fairly open to interfield alignment and solidarity in relation to public health.