Modeling the evaporation and dispersion of airborne sputum droplets expelled from a human cough

This study contributes a new model to simulate the evaporation and dispersion of sputum droplets from human coughs or sneezes. It is the first time different chemical components have been included in order to estimate the transport of sputum or similar biological droplets. This modified model demonstrates the ability to describe real-world phenomena that the widely used single droplet model lacks. Evaporation and dispersion of airborne sputum droplets originating from a human cough are simulated using this model combined with an initially buoyant turbulent jet. Constituents of sputum droplets such as NaCl, amino acids, carbohydrates, and lipids are included. Effects of these chemical components on evaporation rate, velocity, and temperature of droplets are investigated in detail. The results obtained for sputum droplets will provide a perspective of what conditions the viruses within a droplet might face upon being ejected from the mouth during a cough. Finally, computational fluid dynamics (CFD) and probability density function (PDF) techniques were used to complement the new model with a simulation of a cough jet and the dynamics of droplet nuclei in confined spaces. Numerical results indicate that a 10 microns sputum droplet will evaporate to become a droplet nucleus (3.5 microns) in 0.55 s at 0.8 or 80% RH, in 0.3 s at 0.5 or 50% RH, and in 0.25 s at 0.2 or 20% RH. The droplet temperature decreases rapidly from human body temperature to room temperature, which may affect the viability of any carried virus.     

Flow dynamics and characterization of a cough

Abstract Airborne disease transmission has always been a topic of wide interests in various fields for decades. Cough is found to be one of the prime sources of airborne diseases as it has high velocity and large quantity of droplets. To understand and characterize the flow dynamics of a cough can help to control the airborne disease transmission. This study has measured flow dynamics of coughs with human subjects. The flow rate variation of a cough with time can be represented as a combination of gamma‐probability‐distribution functions. The variables needed to define the gamma‐probability‐distribution functions can be represented by some medical parameters. A robust multiple linear regression analysis indicated that these medical parameters can be obtained from the physiological details of a person. However, the jet direction and mouth opening area during a cough seemed not related to the physiological parameters of the human subjects. Combining the flow characteristics reported in this study with appropriate virus and droplet distribution information, the infectious source strength by coughing can be evaluated.

Practical Implications

There is a clear need for the scientific community to accurately predict and control the transmission of airborne diseases. Transportation of airborne viruses is often predicted using Computational Fluid Dynamics (CFD) simulations. CFD simulations are inexpensive but need accurate source boundary conditions for the precise prediction of disease transmission. Cough is found to be the prime source for generating infectious viruses. The present study was designed to develop an accurate source model to define thermo‐fluid boundary conditions for a cough. The model can aid in accurately predicting the disease transmission in various indoor environments, such as aircraft cabins, office spaces and hospitals.     

Respiratory bioaerosol deposition from a cough and recovery of viable viruses on nearby seats in a cabin environment

Respiratory bioaerosol deposition in public transport cabins is critical for risk analysis and control of contact transmission. In this work, we built a two-row four-seat setup and an air duct system to simulate a cabin environment. A thermal manikin on the rear left-hand seat was taken as the infected passenger (IP) and “coughed” three times through a cough generator. The deposited viruses and droplets on nearby seats were measured by a cultivation method and microscope, respectively. The effects of seat backrest and overhead gasper jet were studied. Results showed that the number of deposited virus on the front seat was one order of magnitude higher than that on other seats which only contained droplets smaller than 10 µm in diameter. When the backrest was 15 cm higher than the cough, the deposited number of viruses was reduced to 5% of that with the backrest at the same height with the cough. The gasper jet above the IP with a velocity of 1.5 m/s can reduce the deposited viruses to 4% of that with gasper off. It indicates that both the gasper jet and backrest can work as mitigation measures to block the cough jet and protect the nearby passengers.     

Evaporation and dispersion of respiratory droplets from coughing

Understanding how respiratory droplets become droplet nuclei and their dispersion is essential for understanding the mechanisms and control of disease transmission via droplet‐borne and airborne routes. A theoretical model was developed to estimate the size of droplet nuclei and their dispersion as a function of the ambient humidity and droplet composition. The model‐predicted dried droplet nuclei size was 32% of the original diameter, which agrees with the maximum residue size in the classic study by Duguid, 1946, Edinburg Med. J., 52 , 335 and the validation experiment in this study, but is smaller than the 50% size predicted by Nicas et al., 2005, J. Occup. Environ. Hyg., 2 , 143. The droplet nuclei size at a relative humidity of 90% (25°C) could be 30% larger than the size of the same droplet at a relative humidity of less than 67.3% (25°C). The trajectories of respiratory droplets in a cough jet are significantly affected by turbulence, which promotes the wide dispersion of droplets. We found that medium‐sized droplets (e.g., 60 μm) are more influenced by humidity than are smaller and larger droplets, while large droplets (≥100 μm), whose travel is less influenced by humidity, quickly settle out of the jet.     

Effect of a novel temperature‐controlled laminar airflow device on personal breathing zone aeroallergen exposure

Temperature‐controlled laminar airflow improves symptoms in atopic asthmatics, but its effects on personal allergen exposure are unknown. We aimed to evaluate its effects on personal cat allergen and particulate exposures in a simulated bedroom environment. Five healthy volunteers lay under an active and an inactive temperature‐controlled laminar airflow device for 175 min, in a simulated bedroom containing bedding from a cat owner. Total airborne particles (≥0.5 – ≥10 μm diameter) were quantified with a laser particle counter. Airborne allergen was sampled with Institute of Occupational Medicine filters. Inhaled exposure was sampled with nasal air samplers. Allergen‐containing particles were quantified by immunoassay. Treatment reduced total airborne particles (>0.5 μm diameter) by >99% (P < 0.001) and reduced airborne allergen concentration within the breathing zone (ratio of median counts = 30, P = 0.043). Treatment reduced inhaled allergen (ratio of median counts = 7, P = 0.043). Treatment was not associated with a change in airborne allergen concentration outside of the breathing zone (P = 0.160). Temperature‐controlled laminar airflow treatment of individuals in an allergen‐rich experimental environment results in significant reductions in breathing zone allergenic and non‐allergenic particle exposure, and in inhaled cat allergen exposure. These findings may explain the clinical benefits of temperature‐controlled laminar airflow.     

CFD analysis of a realistic reduced-scale modeling equipped with axial jet fan

Typically, in the experimental scale road tunnel model, the air flow induced by ventilation system is provided by an external fan. In this paper, the authors have numerically simulated full and reduced-scale tunnel in order to evaluate the possibility to realize a reduced scale of a road tunnel model with a realistic ventilation system consisting of impulsive jet fans.In particular, two different types of longitudinal ventilation systems were considered, traditional and alternative. The last one was equipped with jet fans that have the inlet/outlet sections inclined at a fixed pitch angle (α=6°) toward the tunnel floor. The jet fan was simulated as a simple momentum source that provides a pressure rise (pressure drop) across them as a function of the outflow air velocity.The analyzed tunnel consists in a 800 m one directional bore with circular cross section 5.05 m radius; the jet fans were installed at 5.67 m from the floor. Furthermore a burning Heavy Good Vehicle (HGV), placed at 450 m far away the tunnel entrance, was considered. To simulate numerically the burning vehicle, the species transport equation combustion model with Eddy-Dissipation-Concept (EDC) model was adopted.In order to create a reduced-scale model from a full scale, Froude method was applied to preserve geometrical, kinematical and dynamical similitude. Temperature and axial velocity profiles, in different tunnel sections for both considered models (full and scaled) and ventilation systems, were provided. The numerical results showed a good agreement for the both ventilation systems.     

Simplified models for exhaled airflow from a cough with the mouth covered

Covering a cough can be useful in reducing the transmission of airborne infectious diseases. However, no simple method is available in the literature for predicting the exhaled airflow from a cough with the mouth covered. This investigation used smoke to visualize the airflow exhaled by 16 human subjects. Their mouths were covered by a tissue, a cupped hand, a fist, and an elbow with and without a sleeve. This study then developed simplified models for predicting the airflow on the basis of the smoke visualization data. In addition, this investigation performed numerical simulations to assess the influence of mouth coverings on the receptor's exposure to exhaled particles. It was found that covering a cough with a tissue, a cupped hand, or an elbow can significantly reduce the horizontal velocity and cause the particles to move upward with the thermal plumes generated by a human body. In contrast with an uncovered cough, a covered cough or a cough with the head turned away may prevent direct exposure.     

A study on ceiling jet characteristics in an inclined tunnel

The characteristics of a ceiling jet of an inclined tunnel in a fire will be studied and reported in this paper. Scale modeling experiments on a ceiling jet in a model tunnel of length 3.0 m, width 0.8 m and height 1.0 m inclined at different angles of 0°, 10°, 20° and 30° were carried out. Numerical studies by large eddy simulation were then performed. Both experimental observation and numerical simulation indicated that the characteristics of the temperature and velocity fields near the upper tunnel are different from those obtained using the empirical equations reported in the literature. Another set of empirical equations for gas temperature and flow velocity along the tunnel were fitted by experimental data. These derived empirical equations are useful for estimating the temperature and flow velocity patterns for the ceiling jet in an inclined tunnel with an angle within the range 0–30°.     

Measurements of spray-plume interactions for model validation

Fire suppression using automatic fire sprinklers is tremendously successful in reducing loss of life and property in the event of a fire. With the increasing computing power available, as well as the spread of performance-based design methods, the ability to accurately model spray dispersion and suppression is desirable. In this study, experiments were conducted to quantify spray dispersion and spray-plume interactions for model validation. Numerical simulations of these spray interactions were performed using FireFOAM. These simulations were distinguished by the use of comprehensive highly-resolved initial spray measurements to generate the numerical spray. The experimental Sprinkler Array Facility (SAF) used in this study consisted of a centrally located, well-characterized, forced air jet (simulating the updraft from a real fire plume) providing a challenge to the spray. Reliable model boundary conditions were established from detailed measurements of the air jet injection velocities and detailed measurements of the initial spray using the Spatially-resolved Spray Scanning System (4 S). Measurements of volume flux as well as optical measurements of drop size and velocity were obtained at various locations within the air jet. Four flow conditions were investigated with the intent of providing model validation data; close and far sprinkler spacing, each with quiescent air and strong jet conditions. The strong jet was capable of overwhelming the smallest drops within the spray, reversing their direction, and reducing the volume flux at the floor. Computational simulations (informed by detailed initial spray measurements) demonstrated good agreement with the spray dispersion and plume penetration experiments.     

Scale similarity on ceiling jet flow

In this study, empirical formulae previously derived for describing the decrease in temperature rise, the decrease in velocity, the thermal boundary layer thickness, the momentum boundary layer thickness, the Gaussian thermal thickness, and the Gaussian momentum thickness of a ceiling jet flowing upward along the steepest run of an inclined ceiling were applied to a full-scale scenario. The coefficients in these formulae were determined through a series of pool fire tests conducted using a flat, unconfined model ceiling with dimensions of 2.5 m×3.0 m, and fixed ceiling clearance of 1.0 m. To verify the applicability of the developed formulae to actual fires, another series of pool fire tests were conducted using a flat, unconfined full-scale ceiling with dimensions of 7.0 m×14.0 m and a maximum ceiling clearance of 3.0 m. The proposed formulae were confirmed to be applicable to a full-scale scenario and to describe the ceiling jet flow accurately.     

Uniformity of stratum‐ventilated thermal environment and thermal sensation

Three human test series were conducted to evaluate the uniformity of the thermal environments in a stratum‐ventilated chamber with dimensions of 8.8 m (L) × 5.1 m (W) × 2.4 m (H). In all, nineteen conditions were generated by adjusting the room temperature, supply airflow rate, and supply terminal type. An air diffuser performance index (ADPI) of at least 80% was achieved for most cases. This result shows that the air velocity and temperature in the occupied zone are reasonably uniform. Subjective assessments using the ASHRAE 7‐point scale indicate that the thermal sensations of the subjects in stratum ventilation are also uniform. This study examines the applicability of the predicted mean vote (PMV) model for evaluating stratum ventilation. When compared to the actual mean thermal sensation votes (ATS), the PMV values are acceptable. The PMV results at a height of 1.1 m above the floor show better agreement with the ATS than at a height of 0.1 m.     

Transport of exhaled particulate matter in airborne infection isolation rooms

The goal of this research was to examine the characteristics of the spatial velocity and concentration profiles which might result in health care workers’ exposure to a pathogenic agent in an airborne infection isolation room (AIIR). Computational fluid dynamics simulations were performed for this purpose. This investigation expanded on the work of Huang and Tsao [The influence of air motion on bacteria removal in negative pressure isolation rooms. HVAC & R Research 2005; 11: 563–85], who studied how ventilation conditions impact dispersion of pathogenic nuclei in an AIIR by investigating the airflow conditions impacting dispersion of infectious agents in the AIIR. The work included a careful quality assurance study of the computed airflow, and final simulations were performed on a fine tetrahedral mesh with approximately 1.3×10⁶ cells. The 1 μm diameter particles were released from a 0.001225 m² area representing the nose and mouth. Two cases were investigated during the current study: continuous exhalation of pathogen-laden air from the patient and expulsion of pathogenic particles by a single cough or sneeze. Slow decay of particle concentration in the AIIR during the single cough/sneeze simulation and tendency for particle accumulation near the AIIR walls observed in the continuous breathing simulation suggest that unintended exposures are possible despite the ventilation system. Based on these findings, it is recommended that extra care be taken to assure proper functionality of personal protective equipment used in an AIIR.     

What is the relationship between indoor air quality parameters and airborne microorganisms in hospital environments? A systematic review and meta-analysis

Airborne microorganisms in hospitals have been associated with several hospital-acquired infections (HAIs), and various measures of indoor air quality (IAQ) parameters such as temperature, relative humidity, carbon dioxide (CO₂), particle mass concentration, and particle size have been linked to pathogen survival or mitigation of pathogen spread. To investigate whether there are quantitative relationships between the concentration of airborne microorganisms and the IAQ in the hospital environment. Web of Science, Scopus and PubMed databases were searched for studies reporting airborne microbial levels and any IAQ parameter(s) in hospital environments, from database inception to October 2020. Pooled effect estimates were determined via random-effects models. Seventeen of 654 studies were eligible for the meta-analysis. The concentration of airborne microbial measured as aerobic colony count (ACC) was significantly correlated with temperature (r = 0.25 [95% CI = 0.06–0.42], p = 0.01), CO₂ concentration (r = 0.53 [95% CI = 0.40–0.64], p ˂ 0.001), particle mass concentration (≤5 µg/m³; r = 0.40 [95% CI = 0.04–0.66], p = 0.03), and particle size (≤5 and ˃5 µm), (r = 0.51 [95% CI = 0.12–0.77], p = 0.01 and r = 0.55 [95% CI = 0.20–0.78], p = 0.003), respectively, while not being significantly correlated with relative humidity or particulate matter of size >5 µm. Conversely, airborne total fungi (TF) were not significantly correlated with temperature, relative humidity, or CO₂ level. However, there was a significant weak correlation between ACC and TF (r = 0.31 [95% CI = 0.07–0.52], p = 0.013). Although significant correlations exist between ACC and IAQ parameters, the relationship is not definitive; the IAQ parameters may affect the microorganisms but are not responsible for the presence of airborne microorganisms. Environmental parameters could be related to the generating source, survival, dispersion, and deposition rate of microorganisms. Future studies should record IAQ parameters and factors such as healthcare worker presence and the activities carried out such as cleaning, sanitizing, and disinfection protocols. Foot traffic would influence both the generation of microorganisms and their deposition rate onto surfaces in the hospital environment. These data would inform models to improve the understanding of the likely concentration of airborne microorganisms and provide an alternative approach for real-time monitoring of the healthcare environment.     

Influence of pulmonary ventilation rate and breathing cycle period on the risk of cross‐infection

This study examined the characteristics of the exhaled airflow pattern and breathing cycle period of human subjects and evaluated the influence of pulmonary ventilation rate and breathing cycle period on the risk of cross‐infection. Measurements with five human subjects and a breathing thermal manikin were performed, and the peak exhaled airflow velocity from the mouth and the breathing cycle period were measured. Experiments on cross‐infection between two breathing thermal manikins were then conducted in a full‐scale test room, in which the pulmonary ventilation rate and breathing cycle period were varied systematically. Both peak flow velocity and breathing cycle length varied considerably between different subjects. The breathing cycle period in a standing posture was 18.9% lower than in a sitting posture. The influence of pulmonary ventilation rate and breathing cycle period extended up to a separation distance of 1.0 m between the two manikins. Increasing the pulmonary ventilation rate of the exposed person greatly increased the risk of cross‐infection. Decreasing the breathing cycle period from the widely used “6 second” value led to a considerable increase in the risk of cross‐infection. Standing posture resulted in a higher risk of cross‐infection than sitting posture.     

Self induced buoyant blow off in upward flame spread on thin solid fuels

Upward flame spread experiments were conducted on long thin composite fabric fuels made of 75% cotton and 25% fiberglass of various widths between 2 and 8.8 cm and lengths greater than 1.5 m. Symmetric ignition at the bottom edge of the fuel resulted in two sided upward flame growth initially. As flame grew to a critical length (15–30 cm depending on sample width) fluctuation or instability of the flame base was observed. For samples 5 cm or less in width, this instability lead to flame blow off on one side of the sample (can be either side in repeated tests). The remaining flame on the other side would quickly shrink in length and spread all the way to the end of the sample with a constant limiting length and steady spread rate. Flame blow off from the increased buoyancy induced air velocity (at the flame base) with increasing flame length is proposed as the mechanism for this interesting phenomenon. Experimental details and the proposed explanation, including sample width effect, are offered in the paper.     

Airborne fungal cell fragments in homes in relation to total fungal biomass

Fungal exposure may induce respiratory symptoms. The causative agents are compounds in the fungal cell wall. Fragments of microbes may be present in air samples but are not measurable using conventional spore counting or by the determination of viable organisms. This study assesses the proportion of fungal cell biomass and endotoxin in different particle size fractions in air samples from homes. Air samples were collected from 15 homes using a cyclone sampler, collecting particles in three aerodynamic size fractions: <1.0, 1.0–1.8, and >1.8 μm. N‐Acetylhexosaminidase (NAHA) was determined as a marker of fungal cell biomass. Endotoxin was determined using the Limulus amebocyte lysate method. NAHA and endotoxin in the size range <1.0 μm comprised up to 63% (mean 22.7%) and 96.3% (mean 22.6%) of the total concentrations, respectively. There were significant relationships between the amounts of NAHA and endotoxin in the total amount and in the size fraction >1.8 μm but not in the smaller fractions. The results demonstrate significant amounts of fungal cell biomass and endotoxin in particles <1.0 μm. Homes with reported mold damage had a lower concentration of NAHA in particles <1.0 μm than homes without mold damage. To assess airborne exposure for diagnostic and preventive purposes, measurement techniques that include this fraction should be considered.     

Fire spread experiment across Mediterranean shrub: Influence of wind on flame front properties

A fire spread experiment was conducted in the field under wind-blown conditions. The fuel consists of tall and dense Mediterranean shrub vegetation. The plot area was about 30 m wide and 80 m long. This experiment was conducted not only in order to increase the knowledge and understanding of the fire behaviour in the field but to provide data for the validation of physics based models of fire spread. In particular, the effects of wind on the geometric and thermal properties of the flame front in the field were investigated. The flame temperature along the vertical direction and the radiation emitted ahead of the flame front, were measured. The methodology employed in this experiment and some quantitative measurements of wind velocity and direction, flame geometric properties, are also presented and discussed. The measurements and observations exhibit that the behaviour of the fire and the flame structure character are very different from the one encountered at laboratory scale. These preliminary results show that large scale turbulence influence fire spread and affects the flame shape, temperature and radiation emission.     

Temperature and velocity distributions from numerical simulations of ceiling jets under unconfined,inclined ceilings

Numerical simulations of ceiling jets under unconfined, inclined ceilings were conducted with the open-source code FireFOAM. A range of ceiling inclinations, 0–30° was considered with a 14 kW convective heat release rate (HRR) heptane fire used as the plume source, and the ceiling mid-point clearance from the top of the 0.228 m diameter burner kept fixed at 0.89 m. The predicted temperature and velocity in the developing ceiling jets were compared against the experimental data and empirical correlations. Temperature and velocity predictions on the elevated side of the ceiling are in general agreement with experimental data. Flow reversal in the lower side of the ceiling was predicted with good confidence, and comparison with experimental data was found to be reasonable. Following existing convention in the literature, the predicted results were non-dimensionalized using the convective HRR, ceiling height and radial distance from the ceiling mid-point. Comparison of the non-dimensional data on the elevated ceiling side showed better agreement for temperature against the correlation, whereas predicted velocity data showed a wider spread around the correlation values.     

Numerical comparison of performance between traditional and alternative jet fans in tiled tunnel in emergency ventilation

In this paper a computational study was carried out to evaluate the performance of longitudinal ventilation system equipped with an alternative jet fan with respect to traditional one in case of fire in tiled tunnel. The alternative jet fan is equipped with inclined silencers (pitch angle α = 6°) in order to reduce the Coanda effect and consequently shear stress on the tunnel ceiling. The fire was simulated setting heat flux on HGV surface. Computational fluid dynamic analysis was applied to simulate the ventilation in the unidirectional tunnel through κ–ɛ model. The comparison conducted in terms of total thrust required to prevent back-layering phenomena and numerical results were provided in terms of thrust of jet fan values, average velocity values and temperature profiles, for different tunnel slope values. Furthermore the authors have compared the critical velocity provided by CFD analysis with critical velocity provided in the literature.     

Temperature and velocity properties of a ceiling jet impinging on an unconfined inclined ceiling

Understanding the characteristics of ceiling jet flow is important because most fire detectors and suppression devices are designed to operate within the ceiling jet; the increases in temperature and smoke concentration within the ceiling jet become trigger occupants to begin fire-fighting action or to evacuation. A series of pool fire tests was conducted using a flat, unconfined model ceiling with dimensions of 2.5 m (D)×3.0 m (L) and changing the ceiling inclination angle of up to 40°. A single ceiling height is used. Two fire heat release rates were used to evaluate the effects: one with and the other without the flame tip touching the inclined ceiling under a steady-state condition. Maximum temperature and its position were determined based on the measurement using a rake consisting of 0.2-mm-diameter chromel–alumel thermocouples. The maximum velocity and its position were obtained by the particle image velocimetry method. These data were compared with the velocities obtained using a bi-directional flow probe and the relationship between them was clarified. Empirical formulae for the temperature rise and velocity versus the radial distance from the plume impingement point along the steepest run in the upward direction were developed considering the effect of the inclination angle. Variations in the Froude number and the Richardson number with radial distance were clarified with and without the flame tip touching the inclined ceiling.