High-fidelity modeling of the drying kinetics, lifetimes, and trajectories of saliva droplets
Final Report Abstract
Worldwide social distancing rules, designed to prevent the airborne spread of SARS-CoV-2, are based on the idea that small saliva droplets evaporate quickly, while large droplets carrying most viral copies fall fast to the ground. Contrary to the classical perception, saliva droplets do not evaporate entirely but form solid nuclei, significantly increasing their lifetime. Furthermore, solutes might distribute unevenly within the droplets, eventually forming a solid layer at the surface. Acknowledging the lack of knowledge regarding the formation of saliva particles and their transport, we have developed a detailed model tailored specifically for the drying process of saliva droplets. This model has been integrated into a simulation tool, enabling the analysis of individual droplet trajectories after a cough in various ambient conditions. The project’s findings shed light on airborne saliva transmission dynamics, revealing that dried saliva droplets might form hollow particles, significantly impacting their trajectories. This discovery challenges existing models that overlook the non-uniform distribution of non-volatiles, leading to imprecise predictions regarding the droplets’ size evolutions and trajectories. Hollow particle formation occurs in humidity below 80% and is more pronounced in hot and dry environments. On the contrary, in cold and humid conditions, they fall to the ground before a shell forms. Unlike existing models’ predictions, droplets forming hollow particles remain suspended longer, enabling them to cross larger distances. Furthermore, the study shows that the SARS-CoV-2 transmission risk depends not only on the falling time and traveling distance of saliva droplets but also on changes in virus infectivity. Droplets may suspend longer in warm and dry environments, but the virus’s infectiousness diminishes. Conversely, the virus is more infectious in cold and dry environments, but droplets fall quickly to the ground. For the general public, these findings directly impact protective measures. The study indicates that wearing facial masks could provide better protection than relying solely on social distancing. Recognizing the intricate process of droplet drying emphasizes the need for tailored approaches to protective measures. Moreover, the study highlights the benefits of advanced mathematical models to evaluate transmission risks in different settings, indoors or outdoors, providing valuable insights for developing effective strategies for containing future pandemics.
Publications
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Airborne virus transmission: Increased spreading due to formation of hollow particles. Environmental Research, 237, 116953.
Ozler, Gizem & Grosshans, Holger
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High-fidelity modeling of the drying kinetics and lifetimes of saliva droplets in airborne transmission of COVID-19, 11th International Conference on Multiphase Flow, ICMF 2023, April 2-7, 2023, Kobe, Japan
Ozler, G. & Grosshans, H.
