Extended lifetime of respiratory droplets in a turbulent vapour puff and its implications on airborne disease transmission

Abstract

To mitigate the COVID-19 pandemic, it is key to slow down the spreading of the life-threatening coronavirus (SARS-CoV-2). This spreading mainly occurs through virus-laden droplets expelled at speaking, screaming, shouting, singing, coughing, sneezing, or even breathing [1-7]. To reduce infections through such respiratory droplets, authorities all over the world have introduced the so-called "2-meter distance rule" or "6-foot rule". However, there is increasing empirical evidence, e.g. through the analysis of super-spreading events [6, 8-11], that airborne transmission of the coronavirus over much larger distances plays a major role [1-3, 7, 12-15], with tremendous implications for the risk assessment of coronavirus transmission. It is key to better and fundamentally understand the environmental ambient conditions under which airborne transmission of the coronavirus is likely to occur, in order to be able to control and adapt them. Here we employ direct numerical simulations of a typical respiratory aerosol in a turbulent jet of the respiratory event within a Lagrangian-Eulerian approach [16-18] with 5000 droplets, coupled to the ambient velocity, temperature, and humidity fields to allow for exchange of mass and heat [19] and to realistically account for the droplet evaporation under different ambient conditions. We found that for an ambient relative humidity of 50% the lifetime of the smallest droplets of our study with initial diameter of 10 μm gets extended by a factor of more than 30 as compared to what is suggested by the classical picture of Wells [20, 21], due to collective effects during droplet evaporation and the role of the respiratory humidity [22], while the larger droplets basically behave ballistically. With increasing ambient relative humidity the extension of the lifetimes of the small droplets further increases and goes up to 150 times for 90% relative humidity, implying more than two meters advection range of the respiratory droplets within one second. Smaller droplets live even longer and travel further. Our results may explain why COVID-19 superspreading events can occur for large ambient relative humidity such as in cooled-down meat-processing plants [10] or in pubs with poor ventilation. We anticipate our tool and approach to be starting points for larger parameter studies and for optimizing ventilation and indoor humidity controlling concepts, which in the upcoming autumn and winter both will be key in mitigating the COVID-19 pandemic.

Structural stability of SARS-CoV-2 degrades with temperature

“…It is hard to estimate how all individual contributing factors would contribute to the epidemiological picture on the ground. Nonetheless, our findings draw parallels between the stability of SARS-CoV-2 and the original SARS viruses [15] and add to a growing body of research suggesting more viral spread is likely at lower temperatures via a variety of possible contributing factors [6]. A small heater was placed in the enclosure to increase the temperature of the AFM chamber.…”

Modes of Transmission of Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) and Factors Influencing on the Airborne Transmission: A Review

“…Respiratory transmission of the SARS-CoV-2 can occur by droplet transmissions [ 5 , 22 , 23 , 24 , 32 ]. Transmission via droplets happens when carriers of the virus talking, coughing, breathing, singing, and sneezing emit bioaerosol particles that can reach other people’s nose, mouth, and eyes causing infection ( Figure 2 ) [ 24 ].…”

“…In addition, Holland et al reported that, similar to influenza, mumps, Haemophilus influenzae , whooping cough, and rubella, coronaviruses are transmitted by bioaerosol droplets [ 24 ]. Furthermore, Abkarian et al and Chong et al described how flows generated during speaking, singing, breathing, and laughing by asymptomatic and presymptomatic humans contribute to the spread of SARS-CoV-2 virus [ 4 , 5 ]. In addition, experiments and simulations were performed to quantify how exhaled air is transferred during talking and stressed that phonetic characteristics add complexity to the airflow dynamics, i.e., plosive sounds, such as certain words starting with the letter “P” in English, create intense vortical structures that act as “puffs” and quickly reach 1 m distances [ 4 ].…”

“…The rapid outbreak of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that is spread via airborne, droplet, contact, and fecal–oral transmission modes constitutes a worldwide and disruptive challenge to societies [ 1 , 2 ]. Both symptomatic and asymptomatic carriers have been recognized to transmit the SARS-CoV-2 virus via various modes of transmission [ 3 , 4 , 5 ], causing severe respiratory infections among patients receiving medical care in hospitals [ 6 , 7 , 8 , 9 , 10 , 11 ]. Hence, the COVID-19 outbreak has been declared a global public health emergency of international concern by the World Health Organization (WHO) [ 12 , 13 , 14 ].…”

Risk assessment for airborne disease transmission by poly-pathogen aerosols

“…The simulations of Chong et al [23] indicate that 100 µm aerosols are quite ballistic and quickly fall out of the exhaled plume, but 10 µm aerosols are carried along with the plume and stay in the air despite their evaporation being greatly slowed. This suggests that d M should be chosen somewhere in the 10-100 µm range, which is further supported by the Wells curves found by Xie et al [21].…”