Exhaled Droplets Research Articles

Impact of organic compounds on the stability of influenza A virus in deposited 1-μL droplets.

This study investigates how the composition of respiratory fluids affects the stability of viruses in exhaled droplets. Understanding virus stability in droplets is important as it impacts how viruses spread and how we can combat them. We focus on influenza A virus (IAV) and investigate how different organic compounds found in lung fluid and nasal mucus protect the virus from inactivation. We demonstrate that the ratio of organics to salt in the fluid is an indicator of IAV stability. Among organics, small proteins are particularly effective at protecting IAV. Their effect is in part explained by the proteins' influence on the crystallization of salts in the droplets, thereby shielding the viruses from prolonged exposure to harmful salt concentrations. Understanding these mechanisms helps us grasp how viruses sustain their infectivity over time in respiratory droplets, contributing to efforts in controlling infectious diseases.

What sizes of droplets contribute to long-range airborne transmission?

The size range of respiratory droplets contributing to long-range airborne transmission of infections determines the targeted intervention methods. However, the exact size range remains unknown, and the influencing parameters are also undetermined. Here, we investigated the size-resolved transport and fate of respiratory droplets in four reported venues of COVID-19 outbreaks. We utilised a transient number balance model, a set of expired droplet size distributions, existing formulas for size-resolved settling rates and filtration efficiencies, and a deposition model from the International Commission on Radiological Protection. This enabled us to obtain the size-resolved concentrations of exhaled droplets in indoor air, the size-resolved number of droplet nuclei in the inhaled air, and the number of droplets deposited throughout the respiratory tract. The newly defined airborne transmission size range of expired droplets depends on the effective dilution flow rate of the infection venue under consideration. Three criteria were proposed for determining the sizes of droplets involved in long-range airborne transmission. The airborne transmission droplets typically featured an initial diameter of 0.1–4–6 µm, with an hourly volume generation rate of 0.38–0.42 nL/h per index case in the four venues. This newly estimated volume of airborne transmission droplets provides an essential input into the viral load method for estimating the infectious quanta generation rate. Practical significanceOur size-resolved estimation reveals that only a tiny fraction of expired infectious droplets within an airborne transmission size range survives after the removal effects of ventilation, settling, deactivation, and filtration, as well as the transient dilution effect. These droplets remain in indoor air, potentially contributing to long-range airborne transmission. The airborne transmission size range depends on the size-dependent dilution capacity of a room.

Correlation between beverage consumption and droplet production during respiratory activity using interferometric Mie imaging experiment

This study investigates the effects of beverage consumption on droplet production during coughing and speaking. Interferometric Mie imaging (IMI) measures particle size using the diffraction characteristics of light and was used to examine the particle size distribution and particle count concentration of exhaled droplets without water (WW), with still water (SW), and with carbonated water (CW). The parameters of the IMI technique were calibrated using glass beads and respiratory droplets were measured for 16 subjects, which showed that drinking beverages had a significant impact on the particle size distribution during coughing and speaking. Another important aspect of this study was the variability in particle emissions among individuals. The results showed that the consumption of SW and CW led to a significant increase in total particle count concentrations in the coughing condition when compared with WW, with no significant difference among beverage type. Individuals with relatively high particle emissions WW showed more particle generation when consuming SW and CW. When speaking, SW ingestion significantly increased the total particle count concentrations when compared with the WW condition, whereas CW consumption did not increase the total particle count concentrations to the same extent as that in the SW condition. These results emphasize that the consumption of beverages such as SW and CW have the potential to significantly increase particle production during respiratory activities, amplifying the potential risks associated with infection transmission.

Effects of recirculation and air change per hour on COVID-19 transmission in indoor settings: A CFD study with varying HVAC parameters

COVID-19 has already claimed over 7 million lives and has infected over 775 million people globally [1]. SARS-CoV-2, the virus that causes Covid-19, spreads primarily through droplets from infected people's airways, rendering Heating, Ventilation, and Air Conditioning (HVAC) systems critical in controlling infection risk levels in the indoor environment. To understand the dynamics of exhaled droplets and aerosols and the percentage of particles that are inhaled, escaped, recirculated, or trapped on different surfaces for a variety of environmental settings, we have presented our findings from the Computational Fluid Dynamics (CFD) modeling to investigate the impact of changing HVAC parameters in this paper. When combined with the spatial and temporal distribution of droplets, this method can be used to assess the potential risk and strengthen resilience. This finding demonstrates the viability and usefulness of CFD for modeling the distribution and dynamics of droplets and aerosols in confined environments. Our study demonstrates that raising the Air Change per Hour (ACH) from 2 to 8 reduces the risk of particle inhalation by nearly 70 %. Additionally, limiting the amount of air recirculation or increasing the amount of fresh air helps to reduce the number of airborne particles in an indoor space. To reduce the potential for respiratory droplet-related transmission and to provide relevant recommendations to the appropriate authority, the same computational approach could be applied to a wide range of ventilated indoor environments such as public buses, restaurants, exhibitions, and theaters.

In Situ Collection and SERS Detection of Nitrite in Exhalations on Facemasks Based on Wettability Differences.

As one of the common carriers of biological information, along with human urine specimens and blood, exhaled breath condensate (EBC) carries reliable and rich information about the body's metabolism to track human physiological normal/abnormal states and environmental exposures. What is more, EBC has gained extensive attention because of the convenient and nondestructive sampling. Facemasks, which act as a physical filter barrier between human exhaled breath and inhaled substances from the external environment, are safe, noninvasive, and economic devices for direct sampling of human exhaled breath and inhaled substances. Inspired by the ability of fog collection of Namib desert beetle, a strategy for in situ collecting and detecting EBC with surface-enhanced Raman scattering is illustrated. Based on the intrinsic and unique wettability differences between the squares and the surrounding area of the pattern on facemasks, the hydrophilic squares can capture exhaled droplets and spontaneously enrich the analytes and silver nanocubes (AgNCs), resulting in good repeatability in situ detection. Using R6G as the probe molecule, the minimal detectable concentration can reach as low as 10-16 M, and the relative standard deviation is less than 7%. This proves that this strategy can achieve high detection sensitivity and high detection repeatability. Meanwhile, this strategy is applicable for portable nitrite analysis in EBC and may provide an inspiration for monitoring other biomarkers in EBC.

The fate of an inhaled cigarette puff in the human respiratory tract

Objective Cigarette smoking can lead to a host of adverse health effects such as lung and heart disease. Increased lung cancer risk is associated with inhalation of carcinogens present in a puff of smoke. These carcinogenic compounds deposit in the lung at different sites and trigger a cascade of events leading to adverse outcomes. Understanding the site-specific deposition of various smoke constituents will inform the study of respiratory diseases from cigarette smoking. We previously developed a deposition model for inhalation of aerosol from electronic nicotine delivery systems. In this study, the model was modified to simulate inhalation of cigarette smoke consisting of soluble and insoluble tar, nicotine, and cigarette-specific constituents that are known or possible human carcinogens. Materials and Methods The deposition model was further modified to account for nicotine protonation and other cigarette-specific physics-based mechanisms that affect smoke deposition. Model predictions showed a total respiratory tract uptake in the lung for formaldehyde (99%), nicotine (80%), and benzo[a]pyrene (60%). Results The site of deposition and uptake depended primarily on the constituent’s saturation vapor pressure. High vapor pressure constituents such as formaldehyde were preferentially absorbed in the oral cavity and proximal lung regions, while low vapor pressure constituents such as benzo[a]pyrene were deposited in the deep lung regions. Model predictions of exhaled droplet size, droplet retention, nicotine retention, and uptake of aldehydes compared favorably with experimental data. Conclusion The deposition model can be integrated into exposure assessments and other studies that evaluate potential adverse health effects from cigarette smoking.

Statistical analysis of infectious disease transmission risk based on exhaled respiratory droplet trajectory distribution

In the present work, the risk of infectious disease transmission is evaluated based on a statistical analysis of respiratory droplet trajectory distribution. An analytical model recently developed by the authors allows the prediction of the trajectory and evaporation rate of exhaled droplets. The model is used to collect data from a sampling set of more than twenty thousand droplets distributed over a range of diameters from 0.1 μm to 1 mm for different respiratory scenarios. The analytical tool implements the governing equations of droplet transport, evaporation, energy balance, and chemical composition. It also features a two-dimensional unsteady empirical model of respiratory cloud including momentum dissipation and buoyancy. A discrete random walk approach to simulate the droplet turbulent dispersion, and the randomization of the droplet release within the exhalation period and the mouth cross section area complete the model enabling statistical analyses to be rightly performed. With the due boundary conditions, different types of respiratory events can be modeled easily. With additional information on the exhaled droplet size distribution and viral content, spatial maps of virus concentration are derived and associated with the risk of infectious disease transmission being able to discriminate between various transmission routes such as fomite, airborne, or direct inhalation. Different scenarios are presented including mouth breathing, nose breathing, speaking, coughing, and sneezing. The fluid dynamic behavior of respiratory droplets is explored on a size basis, and the role of ventilation discussed. Risk evaluation provides useful information for a knowledgeable discussion on the prevention needs and means from case to case.

Numerical study on the onset of global-scale flow from individual buoyant plumes: Implications for indoor disease transmission

Transport of exhaled droplets and aerosol suspension is a main route for the transmission of highly infectious respiratory diseases. A poorly ventilated room, where human body heat drives the flow and the pathogen motion, is one such paradigmatic situation with an elevated risk of viral transmission. Here, we report a numerical study on human body heat-driven buoyancy convection in a slender rectangular geometry with the geometric size of 12 × 1 × 3 m3. Using large-scale three-dimensional simulations, we reveal how different spacings between human body heat sources can potentially spread pathogenic species between occupants in a room. Morphological transition in airflow takes place as the distance between human heat sources is varied, which shapes distinct patterns of disease transmission: For sufficiently large distance, individual buoyant plume creates a natural barrier, forming buoyant jets that block suspension spread between occupants. Thermal plumes exhibit significant individual effects. However, for small distances, a collective effect emerges and thermal plumes condense into superstructure, facilitating long-distance suspension transport via crossing between convection rolls. In addition, we quantify the impact of morphological transition on the transport of viral particles by introducing tracer particles. The quantitative analysis shows that under certain critical distances, the infection risk becomes significantly elevated due to this transition and collective behavior. Our findings highlight the importance of reasonable social distancing to reduce indoor cross-transmission of viral particles between people and provide new insights into the hidden transitional behavior of pathogen transmission in indoor environments.

Host and viral determinants of airborne transmission of SARS-CoV-2 in the Syrian hamster

It remains poorly understood how SARS-CoV-2 infection influences the physiological host factors important for aerosol transmission. We assessed breathing pattern, exhaled droplets, and infectious virus after infection with Alpha and Delta variants of concern (VOC) in the Syrian hamster. Both VOCs displayed a confined window of detectable airborne virus (24–48 hr), shorter than compared to oropharyngeal swabs. The loss of airborne shedding was linked to airway constriction resulting in a decrease of fine aerosols (1–10 µm) produced, which are suspected to be the major driver of airborne transmission. Male sex was associated with increased viral replication and virus shedding in the air. Next, we compared the transmission efficiency of both variants and found no significant differences. Transmission efficiency varied mostly among donors, 0–100% (including a superspreading event), and aerosol transmission over multiple chain links was representative of natural heterogeneity of exposure dose and downstream viral kinetics. Co-infection with VOCs only occurred when both viruses were shed by the same donor during an increased exposure timeframe (24–48 hr). This highlights that assessment of host and virus factors resulting in a differential exhaled particle profile is critical for understanding airborne transmission.

Host and viral determinants of airborne transmission of SARS-CoV-2 in the Syrian hamster.

It remains poorly understood how SARS-CoV-2 infection influences the physiological host factors important for aerosol transmission. We assessed breathing pattern, exhaled droplets, and infectious virus after infection with Alpha and Delta variants of concern (VOC) in the Syrian hamster. Both VOCs displayed a confined window of detectable airborne virus (24-48 hr), shorter than compared to oropharyngeal swabs. The loss of airborne shedding was linked to airway constriction resulting in a decrease of fine aerosols (1-10 µm) produced, which are suspected to be the major driver of airborne transmission. Male sex was associated with increased viral replication and virus shedding in the air. Next, we compared the transmission efficiency of both variants and found no significant differences. Transmission efficiency varied mostly among donors, 0-100% (including a superspreading event), and aerosol transmission over multiple chain links was representative of natural heterogeneity of exposure dose and downstream viral kinetics. Co-infection with VOCs only occurred when both viruses were shed by the same donor during an increased exposure timeframe (24-48 hr). This highlights that assessment of host and virus factors resulting in a differential exhaled particle profile is critical for understanding airborne transmission.

Numerical simulation of the transport of breath-exhaled aerosol from manikin with realistic nasal airway over short social distances

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.

Multi-scale analysis of respiratory droplets transport within the breathing cloud

Infectious diseases are transmitted primarily through pathogen laden droplets commonly exhaled during respiratory events such as breathing, coughing, or sneezing. The transport and evaporation of droplets are governed by the fundamental laws of fluid mechanics and convection-diffusion. From these, analytical models can be created helping in better understanding pathogens transmission from a mechanical perspective. The droplet transport within the humid air breath cloud and the local ventilation are crucial for accurately predicting the droplet fate. Different levels of complexity are possible for modelling the breath cloud, from simple 1D models to complex unsteady 3D simulations including discrete phase models for the droplets simulation. The former are too simple to capture the fluid dynamics of intermittent jets in detail, while the latter are too computationally expensive. The current work presents a novel multi-scale approach where an analytical model of the droplet transport and evaporation is coupled to unsteady CFD simulations of warm humid puffs of exhaled air. The proposed model has the advantage of the accuracy of an analytical model and the computational cost of a relatively standard unsteady CFD simulation, and can be used to predict the trajectory of the exhaled droplets for a variety of respiratory events.

Lagrangian Supersaturation Fluctuations at the Cloud Edge.

Evaporation of cloud droplets accelerates when turbulence mixes dry air into the cloud, affecting droplet-size distributions in atmospheric clouds, combustion sprays, and jets of exhaled droplets. The challenge is to model local correlations between droplet numbers, sizes, and supersaturation, which determine supersaturation fluctuations along droplet paths (Lagrangian fluctuations). We derived a statistical model that accounts for these correlations. Its predictions are in quantitative agreement with results of direct numerical simulations, and explain the key mechanisms at play.

Characteristics of human-related droplet dynamic patterns inside urban bus with numerical simulation approach: Impacts of solar radiation, ventilation modes and humidity

Nowadays, urban bus has become one of the closely human-related environments. The exhaled droplets when passengers breath, talk, cough and sneeze will undergo a series of significant dynamic process in the enclosed bus. The characteristics of aerosol dynamic process has been demonstrated to be great impact on aerosol transmission and the occupants’ exposure levels, but has not been fully understood yet. This study analyzed the airflow organization and aerosol transmission during actual commuting period inside urban buses by employing the Eulerian-Lagrangian technique and realizable k-ε turbulence model. Three key factors including solar radiation, ventilation modes and humidity were added to reflect the impacts of the abovementioned parameters on aerosol dynamic and transmission patterns. The results showed that solar radiation will lead to a growth of up to 64.4% of bus interior surface temperature in winter. The air conditioning and ventilation inside the bus can meet the thermal comfort needs of passengers. The droplets start to spread in the entire compartment in 30–90 s and almost affects all the passengers, and those sprayed by standing passengers diffuse faster, affecting 8 passengers around the target passenger within 30 s. When relative humidity (RH) = 90%, the residential particle size after evaporation will be 27.9%–38.38% larger than that under RH = 30% with diameter over 15 μm. The findings in the study should be a guidance when accurately forecasting the aerosol dynamic, transmission, exposure levels and smart control for ventilation system inside transport environment.

Retracted: Fluid Dynamics and Numerical Simulation of Exhaled Droplets Containing Infectious Viruses

Retracted: Fluid Dynamics and Numerical Simulation of Exhaled Droplets Containing Infectious Viruses

Experimental characterization of exhaled flow dynamics of human breathing and vocalization

During the onging pandemic of COVID-19, there are numerous asymptomatic patients who are infectious. The exhaled droplets from their daily respiratory activities like breathing or speaking can be the sources of airborne disease transmission of COVID-19. The understanding of the airflow dynamics of these respiratory activities may be helpful to develop effective measures to prevent and control the spread of the disease. In this study, the exhaled flows from human breathing and vocalization of specific syllables are characterized using particle image velocimetry (PIV) and smoke visualization. The exhaled flow generated by ten phonemes including vowels, fricatives, affricates, plosives and nasals as well as mouth breathing are studied. The dynamic developments of the airflow processes are described by a series of parameters, including peak velocity, peak velocity time, duration time, propagation velocity and distance. Results show that vocalization of affricates and plosives as well as mouth breathing tend to have higher peak velocities and propagation distances. The evolutions of exhaled flows generated from these respiratory processes are found to have different jet structures, which are related to the stroke ratio (L/D). The flow field of a small L/D only has a pair of dominant vortices. Whereas that of a large L/D presents both a pair of dominant vortex ring and a trailing jet. Certain phoneme (e.g., /t/) is found to display a two-stage jet similar to a cough with the starting jet and an interrupted jet. The characterization of human exhaled flow of this study may be helpful to provide basis of CFD simulations and a better understanding of the spread of airborne diseases from human breathing.

A statistical analysis of exhaled respiratory droplet trajectory distribution in view of assessing the risk of infectious disease transmission

In the present work an analytical model of exhaled droplet trajectory and evaporation is employed to perform a preliminary statistical analysis of the droplet spatial distribution. The analysis is carried out for a set of over twenty thousand droplets distributed in the range of diameters from 0.1 μm to 1 mm. The type of respiratory events modelled are mouth breathing and speaking. The analytical model employed has been recently developed by the authors. It implements the fundamental laws of fluid dynamics and convection-diffusion, and features a 2D empirical numerical model of buoyant intermittent jet modelling the exhaled breath cloud. A discrete random walk turbulent dispersion model is also included allowing statistical analysis of the droplet trajectory to be performed. With proper boundary conditions any type of respiratory event can be simulated making the model particularly versatile. From the analysis, spatial virus concentration maps are derived for the largest droplets falling to the ground, and for the smaller droplets remaining airborne in the area in front of an infected emitter and successively uniformly dispersed in the ambient. This allows to estimate the risk of infection by fomite, direct inhalation and indirect inhalation routes, and provide a better understanding of infectious disease transmission risks.

Simulation and Analysis of Safe Social Distance under Infectious Disease Epidemic

<p>In this paper, the CFD method is used to study the horizontal distance of the virus infection between the cover and the open opening in the open space. Comparing the spread of virus droplets carrying different particle sizes, the human body's effect on inhibiting virus spread under different forms of protection was studied, and the diffusion concentration of pollutants in the range of 1m before and after the environmental wind speed was studied. The results show that the droplet particles settle when the particle size is larger than 6×10<sup>-5</sup>m. The KN95 mask can reduce the diffusion distance of exhaled droplets by 50% compared with the working condition without a mask. When the upwind wind speed is greater than or equal to 3m/s, the distance of 1m in front of the human body is set reasonable; regardless of the downwind wind speed, there is a risk of infection directly in front of the human body. When the human body wears a KN95 mask, the distance of 1m is safe when there is no wind; when the wind speed is upwind, it is reasonable for a positive direction of 1m, and there is a risk of infection within 1m behind; when there is a downwind wind speed, the distance of 1m in front should be increased. This article can provide a reference for reducing the risk of infection and determining social distance under different environmental conditions.</p>

Transport and evaporation of exhaled respiratory droplets: An analytical model

An important vector for host-to-host infectious disease transmission is given by the transport of tiny pathogen-laden droplets. These are commonly exhaled by individuals while breathing, speaking, coughing, or sneezing. Depending on their size and ambient conditions, they may follow different paths, either settling on surfaces where the pathogen can be further transmitted by contact, or remaining airborne after evaporation where the pathogen can be inhaled. Our understanding of pathogen transmission from the fluid mechanics perspective is still somewhat limited, especially in quantitative terms. In the current work, starting from the fundamental laws of fluid mechanics and diffusion, a detailed analytical model of droplet transport and evaporation in humid air streams is presented and successfully validated against available data in the literature finding remarkable agreement. The model implements closed-form analytical solutions of the equations of transport, evaporation, and energy balance, and an algebraic model to account for the droplet chemical composition. It also features an analytical model of droplet transport within the buoyant exhaled breath cloud based on momentum conservation addressing both jet and puff phases and is able to handle periodic respiratory events. Turbulent dispersion is modeled with a discrete random walk approach. A simple inhalation model is also proposed. Such a model may help in better understanding droplets' fluid dynamic behavior and may be used to assess the risks associated with pathogen transmission under different scenarios for any type of respiratory event. Overall, the computational cost is relatively low, allowing extensive simulation campaigns to be performed easily.

Numerical study of spread of coughing droplets by human walking and indoor wind environment

Indoor ventilation and human movement are important factors affecting the spread of droplets. In this paper, the effects of indoor wind speed and human movement on the propagation and diffusion of cough ejected droplets are investigated using computational fluid dynamics. The effects of cough non-isothermal jet flow, indoor wind velocity, and human movement on the transmission of exhaled droplets were studied. The droplet size ranges in dynamic indoor environments were redefined based on the risk of droplet spread dispersal. The buoyant flux, gravity, inertial forces, and drag forces that affect the droplet motion were evaluated. The results show that droplet spread above 40 μm is limited by indoor airflow or human-induced wake and settles rapidly under the influence of gravity. The indoor air velocity makes the droplets of less than 40 μm to spread within 4 m in front of the cougher within 18 s. The induced wake allowed the lateral movement of droplets between the coughing person and the moving person, but the diffusion effect was much less than the effect of indoor air velocity. The combined effect of indoor air velocity and human movement makes the indoor flow field very complex. It greatly delays the deposition time of 10–40 μm droplets (2.2 times) and makes droplets below 10 μm to spread rapidly to distant locations by the induced wake, increasing the risk of infection. Further research is required to quantify the influence of parameters, such as the human movement and evaporation of droplets among others in indoor dynamic environment.