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.

Turbulent dispersion of breath by the wind

“…where σ 2 a = a 2 . At very short time, the auto-correlation function departs quadratically from 1 and not linearly as given by the exponential formula (26). This is exactly the behaviour reported both numerically and experimentally 21,23 for the Lagrangian velocity structure function.…”

Face-masks save us from SARS-CoV-2 transmission

“…6 shows the mean risk of infection for the different scenarios and for a duration of 20 min as a function of wet diameter cut-off (the diameter above which the particles are assumed to deposit quickly and do not contribute to airborne infection risk). The first observation that can be made is that if a 5 µm cut-off is considered, the typical cut-off size for aerosols [4,7], risk of infection is below 10% for all the scenarios. However, at cut-off size of 50 µm, which in typical room conditions with w = 4 translates to an equilibrium diameter of 12.5 µm, risk of infection increases significantly for distancing and mixed scenarios, particularly when infectious is speaking.…”

“…There is an ongoing debate about whether COVID-19 is transmitted primarily via aerosols or droplets [5,6]. There is also a longstanding debate about what is meant by aerosols and droplets [7]. At their core, these debates are fueled by our inadequate understanding of how airborne disease transmission works, or simply put, how particles produced in the respiratory tract of the infectious become airborne and enter the respiratory tract of the susceptible.…”

Carbon dioxide, COVID-19 and the importance of restaurant ventilation: a case study from Spain approaching Christmas 2021

“…Almstrand et al, 2010;Bake et al, 2019;Haslbeck et al, 2010;Johnson and Morawska, 2009;Schwarz et al, 2010). Once released into the indoor atmosphere, the potentially infective respiratory pathogens will be carried away from the diseased individual in a buoyant turbulent cloud of gas and particles (Boubouiba, 2021;Jones and Bross, 2015;Lv et al, 2021;Randall et al, 2021). Thus, to minimise respiratory infection risk in public indoor spaces the obvious recommendation is to wear a well-fitted, high quality facemask, and combine this with efficient ventilation that ensures the constant introduction of fresh air from outside.…”