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Building Simulation 2023, 16(5): 781-794 https://doi.org/10.1007/s12273-022-0982-0
Research Article | Issue | Published: 13 March 2023

Tracing the origin of large respiratory droplets by their deposition characteristics inside the respiratory tract during speech

Show Author's Information Yihan Wang1,  Jianjian Wei1( ),  Caroline X. Gao2,  Tao Jin1,  Li Liu3( )
Institute of Refrigeration and Cryogenics, Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province, Zhejiang University, Hangzhou 310000, China
Public Health and Preventive Medicine, Monash University, Melbourne, VIC, 3004, Australia
Department of Building Science, Tsinghua University, Beijing 100084, China
Keywords:
COVID-19, respiratory droplets, deposition, airborne transmission, viral load
Cite this article:
Wang Y, Wei J, Gao CX, et al. Tracing the origin of large respiratory droplets by their deposition characteristics inside the respiratory tract during speech. Building Simulation, 2023, 16(5): 781-794. https://doi.org/10.1007/s12273-022-0982-0
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Abstract Full text About this article

Origin of differently sized respiratory droplets is fundamental for clarifying their viral loads and the sequential transmission mechanism of SARS-CoV-2 in indoor environments. Transient talking activities characterized by low (0.2 L/s), medium (0.9 L/s), and high (1.6 L/s) airflow rates of monosyllabic and successive syllabic vocalizations were investigated by computational fluid dynamics (CFD) simulations based on a real human airway model. SST k–ω model was chosen to predict the airflow field, and the discrete phase model (DPM) was used to calculate the trajectories of droplets within the respiratory tract. The results showed that flow field in the respiratory tract during speech is characterized by a significant laryngeal jet, and bronchi, larynx, and pharynx–larynx junction were main deposition sites for droplets released from the lower respiratory tract or around the vocal cords, and among which, over 90% of droplets over 5 μm released from vocal cords deposited at the larynx and pharynx–larynx junction. Generally, droplets’ deposition fraction increased with their size, and the maximum size of droplets that were able to escape into external environment decreased with the airflow rate. This threshold size for droplets released from the vocal folds was 10–20 μm, while that for droplets released from the bronchi was 5–20 μm under various airflow rates. Besides, successive syllables pronounced at low airflow rates promoted the escape of small droplets, but do not significantly affect the droplet threshold diameter. This study indicates that droplets larger than 20 μm may entirely originate from the oral cavity, where viral loads are lower; it provides a reference for evaluating the relative importance of large-droplet spray and airborne transmission route of COVID-19 and other respiratory infections.


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About this article

Tracing the origin of large respiratory droplets by their deposition characteristics inside the respiratory tract during speech

Show Author's information Yihan Wang1, Jianjian Wei1( ), Caroline X. Gao2, Tao Jin1, Li Liu3( )
Institute of Refrigeration and Cryogenics, Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province, Zhejiang University, Hangzhou 310000, China
Public Health and Preventive Medicine, Monash University, Melbourne, VIC, 3004, Australia
Department of Building Science, Tsinghua University, Beijing 100084, China

Abstract

Origin of differently sized respiratory droplets is fundamental for clarifying their viral loads and the sequential transmission mechanism of SARS-CoV-2 in indoor environments. Transient talking activities characterized by low (0.2 L/s), medium (0.9 L/s), and high (1.6 L/s) airflow rates of monosyllabic and successive syllabic vocalizations were investigated by computational fluid dynamics (CFD) simulations based on a real human airway model. SST k–ω model was chosen to predict the airflow field, and the discrete phase model (DPM) was used to calculate the trajectories of droplets within the respiratory tract. The results showed that flow field in the respiratory tract during speech is characterized by a significant laryngeal jet, and bronchi, larynx, and pharynx–larynx junction were main deposition sites for droplets released from the lower respiratory tract or around the vocal cords, and among which, over 90% of droplets over 5 μm released from vocal cords deposited at the larynx and pharynx–larynx junction. Generally, droplets’ deposition fraction increased with their size, and the maximum size of droplets that were able to escape into external environment decreased with the airflow rate. This threshold size for droplets released from the vocal folds was 10–20 μm, while that for droplets released from the bronchi was 5–20 μm under various airflow rates. Besides, successive syllables pronounced at low airflow rates promoted the escape of small droplets, but do not significantly affect the droplet threshold diameter. This study indicates that droplets larger than 20 μm may entirely originate from the oral cavity, where viral loads are lower; it provides a reference for evaluating the relative importance of large-droplet spray and airborne transmission route of COVID-19 and other respiratory infections.

Keywords: COVID-19, respiratory droplets, deposition, airborne transmission, viral load

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Publication history
Copyright
Acknowledgements

Publication history

Received: 06 August 2022
Revised: 12 December 2022
Accepted: 22 December 2022
Published: 13 March 2023
Issue date: May 2023

Copyright

© Tsinghua University Press 2023

Acknowledgements

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51808488, No. 52178092). We are grateful to Dr. Hui-Ling Yen (The University of Hong Kong) and Dr. Jie Zhou (University College London) for providing insightful comments and suggestions that helped improve this study.

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