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Respiratory infections are currently understood to be caused by pathogens released through the nose or mouth of an infected individual, and subsequently transmitted to susceptible hosts. These pathogens are enclosed in liquid particles that are aerosolized from the respiratory tract during activities such as breathing, speaking, sneezing, and coughing. These particles vary widely in size, ranging from submicron to several microns. While past research has largely overlooked the human respiratory system, recent analysis has revealed that the actual structure of the nasal cavity significantly influences the prediction of aerosol transmission during exhalation. In this study, computational fluid dynamics (CFD) simulations were conducted to analyze the aerosol transmission generated during exhalation from the nasal and oral cavities. Realistic nasal and oral cavity structures were taken into account, and authentic temperature distributions were applied to the surfaces. Additionally, inhalation conditions for susceptible individuals were established to evaluate the risk of inhalation-generated exposure. Through various simulation scenarios, we separately discussed the impact of environmental wind speed, separation distance, and exhalation flow rate. The simulation results indicate that environmental wind amplifies the complexity of the flow field and the transmission and deposition of particles between two individuals. Under ambient wind velocities of 0.5 m/s and 1 m/s, it was observed that over 80% of the particles with a diameter of 1 µm inhaled through the nasal cavity accounted for the total deposition on the infected individual. Furthermore, high exhalation flow rates exhibited higher deposition ratios at close distances, in line with our expectations. Therefore, it is advisable to minimize close contact as much as possible during periods of frequent respiratory infections, and to wear masks in order to reduce the risk of inhalation exposure. Implications : During the activities such as breathing, speaking, sneezing, and coughing, liquid particles containing pathogens are aerosolized from the respiratory tract and are released from nose or mouth through the nebulization. In this study, we investigated the transmission of aerosols from human exhalation in the outdoor environment, innovatively taking the real oral-nasal structure and the active inhalation of vulnerable people into consideration, and explored the human-to-human transmission of respiratory viruses. The results are beneficial for public health assessment and policy development.

This study presents the first known successful in vitro measurement of both deposition fractions (DFs) and spatial deposition patterns of nebulized droplets in a realistic human upper respiratory tract model. The model, based on computed tomography scan data of a healthy adult male, accurately replicates the complex geometries of the oral and pharyngeal regions. Experiments were conducted using three different vibrating mesh nebulizer (VMN) systems at varying inhalation flow rates (15–60 l/min). Fluorescein-tagged saline solutions were nebulized, with deposition patterns—representing the spatial distribution of deposited droplets—visualized through ultraviolet fluorescence and DFs quantified based on the absorbance of the rinsate solution. Results show that the DFs and patterns are significantly influenced by flow rate, with deposition shifting from the oral cavity to the oropharyngeal region as flow rate increases. This study provides new insight into aerosol transport and deposition in realistic human airways and offers valuable data for optimizing nebulizer design and improving aerosol drug delivery efficiency in clinical applications. [ABSTRACT FROM AUTHOR]

The efficient delivery of nicotine in e-cigarette aerosols depends on the deposition efficiency of these aerosols within the respiratory tract. Accurate prediction of the hygroscopic growth of e-cigarette aerosols is pivotal to understanding their deposition. This study introduces a novel experimental approach to quantify the water activity coefficient, which is a crucial parameter governing the extent of hygroscopic growth in e-cigarette aerosols. Propylene glycol (PG) and vegetable glycerin (VG) are the primary components of e-liquid so that are used as the experimental subjects. Our findings reveal intriguing trends in the behavior of PG and VG mixtures. As relative humidity (RH) increases, these mixtures exhibit elevated equilibrium mass fractions of water accompanied by a decrease in water activity coefficients. Moreover, higher initial PG/VG mass ratios lead to decreased mass fractions of water and increased water activity coefficients. Notably, solution pH also impacts water activity coefficients, with higher pH levels corresponding to increased coefficients. The results reveal the factors which can affect the hygroscopic growth of e-cigarette aerosols, thus contributing to the understanding of the deposition of such aerosols within the respiratory tract. Moreover, the experimental apparatus proposed in this study provides a straightforward and reliable method to obtain hygroscopic parameters of multicomponent solutions. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nebulizer is one of inhalation therapy's most widely used aerosol generation devices. Nowadays, the vibrating mesh nebulizer has become popular owing to its compactness and noiselessness. In this study, an experimental system is proposed to measure the deposition fraction (DF) of nebulized sodium chloride (NaCl) droplets in an idealized mouth-throat airway model. The results show that before the DF increases to 58.6% with an increasing flow rate to 60 L/min, there is a decrease in DF from 42.4% to 15.5% when the inhalation flow rate increases from 15 to 22.5 L/min for a normal saline solution. These results substantially differ from the conclusions of dry powder or particle experiments and simulations, which monotonously increases with increasing inhalation flow rate. This suggests that droplet evaporation during aerosol generation and transport in the airway plays an important role. The experiment also showed that droplets generated from solutions with a higher NaCl concentration have higher DFs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Numerically investigated the transport and deposition of the aerosols in an airway. • Considered hygroscopic growth of the aerosols. • Compared different temperature and humidity conditions of the airway. • Discussed the deposition fractions, final locations and sizes of the aerosols. The hygroscopicity of the aerosols could be utilized to enhance the delivery efficiency of the drug aerosols for pulmonary diseases. In a simple mouth-throat airway model, the heat and mass transfer of the airway boundary, i.e., convection over the mucus layer, evaporation heat of the water vapor evaporated from the mucus layer and the conduction in the mucus-tissue region, have been found to have notable impacts on the transport and deposition of hygroscopic aerosols (Chen et al., 2018). The effect of this coupled heat and mass transfer on the hygroscopic aerosol transport is yet unknown in the deeper lung region in light of the increases of the airway temperature and humidity. This numerical study investigates the transport and deposition of the hygroscopic aerosols in a G3-G6 lung airway under three conditions: (i) constant temperature at 37 °C and constant relative humidity (RH) at 0% in the whole airway, i.e., neglecting hygroscopicity; (ii) more realistic boundary including the mass and heat transfer at the airway boundary; (iii) constant temperature at 37 °C and constant RH at 99.5% in the whole airway. Besides the temperature and RH distributions of the airway, the deposition fractions and deposition patterns of the hygroscopic aerosols as well as the distributions of the escaped aerosols are compared. As the most important outcome, the results show that using the heat and mass transfer of the airway boundary can better predict the aerosol hygroscopic growth and thereby more accurately estimate the deposition fraction of the hygroscopic aerosol. Thus, for the design of a high efficiency pulmonary drug delivery system, including the device and the hygroscopic drug aerosol, on a patient-specific level in future, the heat and mass transfer of the airway boundary should be considered. [ABSTRACT FROM AUTHOR]

Hygroscopic growth of inhaled aerosols plays an important role in determining particle trajectories and hence local deposition sites. Accurate predictions of airway temperature and humidity as well as droplet-vapor interaction are critical for the calculation of hygroscopic growth. Employing a simple mouth-throat (MT) model as a computer simulation test bed, the effects of interactive heat transfer between air-droplet flow and mucus-tissue-layer have been analyzed. For a steady inhalation flow rate of 15 L/min, air temperature and relative humidity distributions affecting droplet growth, deposition efficiency (DE), and deposition pattern have been compared for different thermal airway-wall conditions. The effects considered include: (i) the latent heat of mucus-layer evaporation and convection heat transfer; (ii) convection heat transfer only; and (iii) mucus-tissue layer with constant temperature. As the most important outcome, the validated modeling results show that thermal airflow and mucus-layer interaction can significantly reduce hygroscopic growth and thereby decrease the DE of multicomponent droplets up to 10%. The modeling framework presented can be readily expanded to other systems. Copyright © 2018 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

This study employed the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) to investigate targeted droplet delivery in tracheal intubation. Our findings reveal that droplet behavior and deposition patterns are significantly influenced by droplet size. Smaller droplets (3 μm and 5 μm) closely follow airflow, showing similar deposition patterns in both CFD-DEM and CFD-DPM simulations. In contrast, larger droplets (7.5 μm and 10 μm) demonstrate increased deposition, particularly in the bent section of the Point Source Release tube (PSAR tube), due to enhanced inertia following collisions and coalescence. The study highlights that larger droplets experience a higher frequency of coalescence, especially in curved tube sections. This leads to altered droplet size distribution at the outlet, affecting deposition in the lower respiratory tract. These insights are crucial for optimizing inhalation therapy devices, emphasizing the importance of considering droplet interactions in CFD simulations to accurately predict aerosol behavior in pulmonary drug delivery systems. [Display omitted] • Innovative application of CFD-DEM for aerosol dynamics. • Customized collision model for liquid droplets. • Detailed analysis of droplet coalescence. • Quantitative evaluation of coalescence in PSAR tube sections. • Implications for targeted drug delivery in respiratory therapy. [ABSTRACT FROM AUTHOR]

• The evolution, transport and deposition of hygroscopic droplets are simulated in a highly idealized mouth-throat model. • The effects of different nebulization rates on aerosol evolution are considered. • The effects of different coupling methods on predicting droplet evolution are discussed. Inhalation therapy is widely used to treat various respiratory diseases. Nebulizers have been widely adopted due to their ease of operation, high drug delivery efficiency, and low residual volume. Traditional studies on drug delivery in the respiratory tract typically use a one-way coupled method to track drug transport and deposition for non-hygroscopic drug particles. However, neglecting the concentration of nebulized hygroscopic droplet aerosols can lead to significant discrepancies in the deposition efficiency (DE) of nebulized drugs in the airways. To investigate the effect of different nebulization rates on the evolution, transport, and deposition of droplets, a highly idealized mouth-throat (MT) model was selected as a testbed for the in silico trials of drug delivery to the respiratory tract. The interaction between continuous and discrete phases was modeled using a two-way coupled model of highly concentrated droplet aerosol-water vapor interaction, innovated by the particle parcel method. The effects of different coupling methods and boundary conditions on the average droplet diameter and droplet deposition distribution pattern at the outlet of the MT model were compared using a computational fluid dynamics (CFD) method. Simulation results indicate that neglecting the effect of water vapor evaporated from the droplets on the relative humidity (RH) in the oral cavity could lead to a 2.2-fold prediction error of the average droplet diameter at the outlet of the MT model for a conventional nebulization rate of 0.386 g/min. As the nebulization rate increases, the difference in the predicted average droplet diameter at the outlet of the MT model between the one-way coupled method and the two-way coupled method also increases. Increasing the nebulization rate during inhalation therapy could lead to incomplete evaporation of drug droplets, resulting in deposition of larger diameter drug droplets in the oral cavity. In contrast, a lower nebulization rate can reduce the droplet diameter at the outlet of the MT model, allowing it to enter deeper airways. [ABSTRACT FROM AUTHOR]

This study delves into the dynamics of respiratory droplet dispersion in indoor settings, focusing on three pivotal aspects: the use of Respiratory Models (RM) versus Non-Respiratory Models (NRM), the influence of social distancing, and the size of exhaled droplets. Our advanced simulations reveal that RMs are essential for accurately predicting airflow and droplet trajectories, whereas NRMs tend to overestimate deposition. We further explore how variations in the distance between an infector and a susceptible individual significantly affect droplet behavior and potential transmission. Additionally, our findings highlight the critical role of droplet size in determining their travel and deposition patterns. These insights collectively provide a comprehensive understanding of aerosol transmission in indoor environments, offering valuable contributions to public health strategies aimed at controlling airborne diseases. [Display omitted] • Integrating respiratory models: boosts droplet trajectory prediction accuracy. • Simulating exhaled droplets: accounts for evaporation, heat, and mass transfer. • Analyzing social distance impact: varies droplet deposition on susceptible. • Assessing exhalation flow: crucial to determining exhaled droplets' fate. [ABSTRACT FROM AUTHOR]

Inhalation therapy is widely used in the treatment of acute epiglottitis (AE). However, few studies specifically consider the anatomical variations in pediatric AE treatment. This study aims to evaluate the deposition pattern of inhalation corticosteroids (ICS) in realistic upper airway models of pediatric AE patients. The epiglottic angle increases with the severity of AE, and five models of epiglottic angle were constructed. The computational fluid-particle dynamics (CFPD) was employed to simulate the airflow transport of pediatric AE and the particle deposition in the target area. The results show that as AE progressed, worsened airway narrowing was observed in the epiglottis region, accompanied by a decreasing trend in the optimal ICS particle size. The deposition pattern of ICS in pediatric AE is closely linked to the disease severity. Furthermore, the airway resistance increases significantly at 75°AE, providing an aerodynamic basis for assessing the severity of laryngeal obstruction. [Display omitted] • The inhalation of corticosteroids in different cases of acute epiglottitis was conducted virtually. • Pediatric acute epiglottitis with varying severity levels was firstly studied to date. • A substantial increase in airway resistance was observed at 75° acute epiglottitis. • Optimal inhaled corticosteroid particle size negatively correlated with epiglottis swelling. [ABSTRACT FROM AUTHOR]