Summarised from various sources18 30-32
  1. Superspreading events: the virus is often transmitted at mass events from one or a few people to many people.33 34
  2. Long-range transmission: the virus spreads in shared air among people who have never physically met or touched any common surface.35
  3. Asymptomatic and presymptomatic transmission: a high proportion of people who pass on the virus have no symptoms at the time.36-38
  4. Indoor dominance: transmission is many times greater indoors than outdoors,39 and ventilation reduces transmission.40
  5. Nosocomial infections occur despite strict contact-and-droplet precautions, and reduce when airborne precautions are added.41
  6. Whilst SARS-CoV-2 is difficult to isolate from air,42 viable SARS-CoV-2 has been detected in the laboratory43 and in real-world settings where infected people had been.44 45
  7. SARS-CoV-2 has been detected in air filters in building ducts (could only have got there via airborne route).46
  8. Transmission between animals has occurred when their cages were connected via air ducts.47
  9. The virus exhibits overdispersion (one person with Covid-19 may infect no-one; another may infect dozens).48
  10. Empirical evidence supporting droplet or fomite transmission is surprisingly sparse.11 12
Countries such as Japan,21 where aerosol scientists were part of the inside track with the ear of government,13 had introduced airborne precautions early in the pandemic (Box 1). But in most Western countries, the aerosol narrative—at least initially—fell on deaf policy ears.
By July 2020, aerosol scientists were alarmed that official advice was based on over-simplistic and incorrect models of transmission (which had perpetuated for decades in the infection control literature49), and wrote an open letter to the World Health Organisation offering to help.40

“Covid is ‘situationally’ airborne”

The World Health Organisation’s early guidance on protecting healthcare workers from Covid-19 recommended a standard level of protection for most activities but a higher level for so-called “aerosol-generating” ones,50 reflecting a long-established (but perhaps flawed) research tradition.51 Its Infection Prevention and Control Research and Development Expert Group for COVID-19 (IPCRDEG-C19) did not include any aerosol scientists and did not welcome the open offer of help. A new scientific brief was quickly published, reiterating the dominance of droplet transmission in most circumstances but acknowledging airborne transmission in certain situations—aerosol-generating medical procedures and crowded, poorly-ventilated indoor settings.52
This “situationally airborne” narrative has persisted despite evidence against it (next section), and has far-reaching implications. If aerosols transmit only when certain procedures are being performed, only a small fraction of healthcare staff need higher-grade protection, and only when performing particular procedures. If that assumption is incorrect, staff (especially non-medical and less senior ones) and patients in most healthcare facilities are under-protected.53

“Everyone generates aerosols; everyone is vulnerable”

A systematic review revealed wide disagreement among guideline panels about which procedures and activities should count as “aerosol generating” (and hence earn respirator-grade protection for the person doing them).54 Many procedures (e.g. taking a nasopharyngeal swab) were inconsistently classified, some aerosol-generating acts (e.g. coughing) were not procedures, and several procedures were classified as aerosol-generating only because they induced coughing.54
A detailed review of the physiology and aerodynamics of respiratory acts concluded that coughing, sneezing, breathing (especially if laboured), speaking (especially loudly) and singing generated significant amounts of aerosol; well-documented super-spreader events for Covid-19 involved a critical triad of poor ventilation, crowding and loud vocalisation.34
These findings raise some paradigm-challenging questions. Should respirator-grade protection be worn by everyone—including other patients—whenever patients are coughing ? Should more attention be paid to measures higher up the hierarchy of controls, such as ventilation or filtration of air, or ensuring that fewer people share air and for shorter periods5?
In the sections which follow, we consider some dramatic consequences of the government’s decision to deny, dismiss or downplay the importance of airborne transmission of SARS-CoV-2.