Mechanisms controlling the transport and evaporation of human exhaled respiratory droplets containing the severe acute respiratory syndrome coronavirus: a review

Abstract

Transmission of the coronavirus disease 2019 is still ongoing despite mass vaccination, lockdowns, and other drastic measures to control the pandemic. This is due partly to our lack of understanding on the multiphase flow mechanics that control droplet transport and viral transmission dynamics. Various models of droplet evaporation have been reported, yet there is still limited knowledge about the influence of physicochemical parameters on the transport of respiratory droplets carrying the severe acute respiratory syndrome coronavirus 2. Here we review the effects of initial droplet size, environmental conditions, virus mutation, and non-volatile components on droplet evaporation and dispersion, and on virus stability. We present experimental and computational methods to analyze droplet transport, and factors controlling transport and evaporation. Methods include thermal manikins, flow techniques, aerosol-generating techniques, nucleic acid-based assays, antibody-based assays, polymerase chain reaction, loop-mediated isothermal amplification, field-effect transistor-based assay, and discrete and gas-phase modeling. Controlling factors include environmental conditions, turbulence, ventilation, ambient temperature, relative humidity, droplet size distribution, non-volatile components, evaporation and mutation. Current results show that medium-sized droplets, e.g., 50 µm, are sensitive to relative humidity. Medium-sized droplets experience delayed evaporation at high relative humidity, and increase airborne lifetime and travel distance. By contrast, at low relative humidity, medium-sized droplets quickly shrink to droplet nuclei and follow the cough jet. Virus inactivation within a few hours generally occurs at temperatures above 40 °C, and the presence of viral particles in aerosols impedes droplet evaporation.

Keywords: Airborne infection; COVID-19; Expiratory aerosols; Physical distancing; SARS-CoV-2; Virus transmission.

Fig. 1

Main transmission modes of the severe acute respiratory syndrome coronavirus (SARS-CoV-2), based on the classifications by Li (2021) and Priyanka et al. (2020) 1. Susceptible individuals close to an infected person are prone to drop-spray and short-range airborne transmission. 2. Individuals beyond a certain physical distance, e.g., 1.5 m, are still prone to long-range airborne transmission, e.g., by aerosols. 3. Individuals who touch virus-contaminated inanimate objects, i.e., fomites, are prone to indirect contact transmission. The person who engages in direct physical contact with an infected person, e.g., hugging, hand shaking, or kissing, can also be at significant risk of infection by SARS-CoV-2

Fig. 2

Computational turbulence models and profiles of cough turbulent jet/puff: a images extracted for direct numerical simulation (Li et al. 2022a), b large eddy simulation (Liu et al. 2021a), and c Reynolds-averaged Navier–Stokes model (Quiñones et al. 2022). (1) The direct numerical simulation gives a more precise presentation of the chaotic nature of the jet and puff. It also captures the detached chaotic vortexes in visualization A. (2) The large eddy simulation also captures more fluctuations of the cough jet in visualization B and the detached vortexes. (3) Finally, the Reynolds-averaged Navier–Stokes captures the cough jet’s less chaotic nature than the large eddy simulation in visualization C. Reprinted with permission of Elsevier and AIP publishing from Quiñones et al. (2022) and Liu et al. (2021a), respectively

Fig. 3

Maximum horizontal distance of respiratory droplets. Note that E depicts an Experimental study, and M represents a Modeling numerical or mathematical approach. We note that a classical investigation by Wells (1934) predicted a maximum horizontal spread distance of 2 m. In contrast, new research has revealed that, when we consider turbulence and environmental factors, respiratory droplets can spread to even spaces beyond 6 m

Fig. 4

a Entrapped droplets within a turbulent cough puff after 0.54 s time stamp. The ejected puff and droplets are at 35 °C, and the ambient temperature is at 20 °C. The large droplets overshoot the puff while smaller droplets remain afloat for extended durations. b The non-volatile volume of viral droplets is indicated for both dry (dashed orange line) and humid (dashed purple line) conditions. These viral droplets are engulfed within the turbulent puff of the cough. The nearly constant volume of non-volatiles confirms complete evaporation in dry conditions (dash orange line) and its conformity with the overall volume (orange line). Additionally, research shows that the potentially contagious viral components do not deactivate after evaporation. Reprinted with permission from Liu et al. (2021b). Copyright © 2021, the Author(s), licensed under a Creative Commons Attribution (CC BY) license

Fig. 5

Effects of evaporation on spread distance of distinct droplets, i.e., 100 μm, 60 μm and 20 μm, under two relative humidities, i.e., RH = 0% and 50%, and initial salt mass fraction of 0.9%; Data were extracted from Liu et al. (2017) and Xie et al. (2007). Liu et al. (2017) reported that droplets greater than or equal to 80 µm, e.g., 100 µm in this case, always deposited around a horizontal distance of 1 m from the mouth, irrespective of the change in relative humidity, i.e., 0% and 90% in their case. Xie et al. (2007) also reported similar observations. We found that the 100 μm droplets of both studies did not travel beyond 1.5 m under both relative humidities. In contrast, the 60 μm droplet traveled close to 1.8 m at a relative humidity of 50% and beyond the computational domain of 4 m for 0%. For the small droplet of 20 μm, Xie et al. (2007) reported droplet only traveled 1.5 m at 50% relative humidity. In contrast, Liu et al. (2017) found droplets traveling beyond the 4 m computational domain at 0% and 90% relative humidity