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

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. 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, that airborne transmission of the coronavirus over much larger distances plays a major role, 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 with 5000 droplets, coupled to the ambient velocity, temperature, and humidity fields to allow for exchange of mass and heat and to realistically account for the droplet evaporation under different ambient conditions. We found that for an ambient rel- ative 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, due to collective effects during droplet evaporation and the role of the respiratory humidity, while the larger droplets basi- cally 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. We also show that for low ambient temperatures the problem is even more serious, as the humidity saturation level of air goes down with decreasing tempera- ture. Our results may explain why COVID-19 superspreading events can occur for large ambient relative humidity such as in cooled-down meat-processing plants or in pubs with poor ventilation. We anticipate our tool and approach to be start- ing points for larger parameter studies and for optimizing ventilation and indoor humidity controlling concepts, which both will be key in mitigating the COVID-19 pandemic.

This is joint work with Kai Leong Chong, Chong Shen Ng, Naoki Hori, Morgan Li, Rui Yang, and Roberto Verzicco.