Numerical investigations of exhaling respiratory aerosol from inside of the human respiratory tract

Abstract: The characteristics of exhaled aerosol outside the human respiratory airway are of significant importance in understanding virus transmission, yet they remain poorly understood. In order to effectively prevent and control the spread of respiratory infectious diseases, this study numerically investigates the exhaling characteristics of respiratory aerosol exhaled from the bronchus or larynx of a human upper airway model. This is achieved using the Euler-Lagrange method and considering various aerosol diameters ( dp =0.1, 0.3, 0.5, and 1-20 μm) as well as five expiratory flow intensities ( Q =15, 30, 60, 90, and 120 L/min). The important findings of this study are as follows: (1) Expiratory airflow exhibits complex flow phenomena, including jet-flow, flow separations, and vortex structures, with their characteristics being influenced by the expiratory flow intensities. (2) The exhaling characteristics of aerosol vary depending on the combined effects of expiratory flow intensities, aerosol diameters, and initial exhaled locations from either the bronchus or larynx. (3) A critical diameter ( d _c ) is identified to represent the size at which aerosol can effectively exit the respiratory airway and potentially pose a transmission risk. This critical diameter is identical for aerosol exhaled from both the bronchus and larynx under the same expiratory flow intensity, but it decreases as the expiratory flow intensity increases. In conclusion, expiratory flow intensity is the most critical factor in determining whether aerosol droplets can be expelled from the respiratory airway, as well as influencing the critical diameter ( d _c ) for aerosol droplets initially located in/after the larynx.