How did we get here: what are droplets and aerosols and how far do they go? A historical perspective on the transmission of respiratory infectious diseases

Abstract

The COVID-19 pandemic has exposed major gaps in our understanding of the transmission of viruses through the air. These gaps slowed recognition of airborne transmission of the disease, contributed to muddled public health policies and impeded clear messaging on how best to slow transmission of COVID-19. In particular, current recommendations have been based on four tenets: (i) respiratory disease transmission routes can be viewed mostly in a binary manner of 'droplets' versus 'aerosols'; (ii) this dichotomy depends on droplet size alone; (iii) the cut-off size between these routes of transmission is 5 µm; and (iv) there is a dichotomy in the distance at which transmission by each route is relevant. Yet, a relationship between these assertions is not supported by current scientific knowledge. Here, we revisit the historical foundation of these notions, and how they became entangled from the 1800s to today, with a complex interplay among various fields of science and medicine. This journey into the past highlights potential solutions for better collaboration and integration of scientific results into practice for building a more resilient society with more sound, far-sighted and effective public health policies.

Keywords: aerosols; droplets; historical; public health; respiratory diseases; transmission.

Figure 1.

Contributions of key researchers and public health officials in shaping the understanding of respiratory infection transmission and infection control strategies. Mildred Wells and Cretyl Mills, not pictured, also made key contributions to the field.

Figure 2.

Tracing of how the term ‘airborne’ has been used and understood in the literature, beginning with WHO and CDC definitions as used in infection prevention and control (IPC) guidelines as well as in SARS-CoV-2 scientific briefs. A purple node indicates that a source primarily uses a definition of ‘airborne’ that means ‘particles that remain suspended in the air’. A light green node indicates a source that primarily uses a definition of ‘particles that can be inhaled’, and a dark green node indicates a source that primarily uses a definition of ‘particles that are infectious’. The colour of the arrow connecting the nodes indicates which definition the older source is being used to support in the more contemporary source, even if incorrectly. For example, Siegal et al. CDC [49] cite Duguid [51] to support an airborne definition regarding infectiousness, while Duguid [51] understands ‘airborne’ as only what remains suspended in the air. A dotted line indicates the older source is being cited in order to disagree or dismiss the findings of such source, such as Wells [52] and Flügge [–31].

Figure 3.

Tracing of citations of standard droplet infection ranges as used by both the WHO and the CDC. Nodes in blue indicate the use of a 6 foot/2 m designation, while nodes in peach indicate the use of a 3 foot/1 m designation. Sources in red argue for a variable range. Arrows between nodes indicate how an older source is cited in the newer. In some cases, the edge colour matches the node colour (e.g. WHO [48] cites Jefferson et al. [63] to support a 3 foot range) and in others, it does not (e.g. WHO [64] cites Thornburn et al. [65] to support a 3-foot range, but Thornburn et al. [65] do not use that distance, as indicated by its grey node colour). A few key sources in the historical droplet/aerosol conversation are included here despite being outside of the citation trace working backward from WHO/CDC guidelines, both to keep some continuity with figure 2 and to show historical shifts in understanding. For instance, Wells [52] is included, with a dotted edge to both Chapin [66] and Flügge [–31] to indicate disagreement with how both sources understand droplet range.