Xiaojian Xie

The exchange of water vapor between high-concentration droplets and air significantly influences droplet deposition in the upper airway model during nebulizer use. This study employed a two-way coupled Eulerian–Lagrange method to quantify nebulized droplet evaporation and relative humidity (RH) variations within an idealized mouth–throat (MT) airway model, utilizing validated numerical models. The water vapor interaction between high-concentration droplets and inhaled air was computed using a multiplier based on the particle parcel method. Simulations of normal saline droplet flow inhalation in the MT airway were conducted under two environmental conditions: indoor (26.5 °C, RH = 50%) and warm and wet (30 °C, RH = 75%), with various inhalation flow rates mirroring previous experiments. Droplet deposition fractions (DFs) and deposition patterns were recorded. The results indicated that DF initially decreased and then increased with rising inhalation flow rates. The largest discrepancy between predicted and measured DFs was 10.86%. These findings support the theory that the balance between droplet evaporation and elevated air RH dictates the deposition of nebulized droplets in the airway. Additionally, simulations revealed that environmental conditions significantly affect droplet DF, with variations up to 20.78%. The deposition hotspot shifted from the anterior to the posterior pharynx.

Accurate measurement of droplet/particle deposition fraction and spatial distribution is vital for understanding various inhalation processes, including nebulized therapy, inhalation of atmospheric pollutants, and exposure risk assessment to aerosols such as airborne SARS-COV-2-laden droplets. This paper presents a novel method to measure the deposition fraction and spatial distribution of the deposited particulate phase (i.e., deposition pattern) of droplets through a single experiment. The experiment employs an idealized mouth-to-throat model as a test platform for two vibrating mesh nebulizers to deposit droplets. By utilizing a nebulized solution of normal saline containing the fluorescein, the qualitative observation of droplet deposition pattern on the internal surfaces is achieved under ultraviolet excitation. Furthermore, through rinsing the experimental components and quantitatively determining the deposition fraction based on rinsate absorbance, experimental results indicate that the deposition fraction of nebulized droplets decreases initially and then increases with increasing inspiratory flow rate from 15 to 60 l/min. Additionally, the deposition hotspots gradually shift from the bottom of the oral cavity to the throat as the inhalation flow rate increases. In addition to providing validation data for the transport and deposition of high-concentration droplets, this experimental method has the potential for extension to research on aerosol transmission and exposure risk assessment. It offers valuable insight into the behavior of nebulized droplets, aiding in developing effective strategies for aerosolized therapies and mitigating transmission risks in various applications.

Basic analysis of the flow field and aerosol deposition under different conditions when a spreader contains an upper airway tract is important to accurately predict the transmission of virus-laden aerosols. An upper airway was included to simulate aerosol transport and deposition. A flow field was simulated by the Transition SST model for validation. The simulation results show that, in the absence of the upper airway structure, an over-predicted aerosol deposition rate will occur. Higher upper-stream air velocity enhanced the intensity but added complexity to the recirculating flow between two manikins and increased the deposition rate of aerosol in the disseminator. A low-temperature environment can reduce the deposition rate of aerosol particles on the body of the disseminator due to a strong thermal plume. Therefore, the structure of the upper airway should be considered when predicting respiratory aerosol in order to increase the accuracy of aerosol propagation prediction.

I totally agree with the commentary. The public should pay more attention on indoor air quality and ventilation. Carbon dioxide monitoring is a convenient and inexpensive method, and should be promoted in public space. More data on the relation of ca...