M.P. Wan

Characterization of expiration air jets and droplet size distributions immediately at the mouth opening

Abstract Size distributions of expiratory droplets expelled during coughing and speaking and the velocities of the expiration air jets of healthy volunteers were measured. Droplet size was measured using the interferometric Mie imaging (IMI) technique while the particle image velocimetry (PIV) technique was used for measuring air velocity. These techniques allowed measurements in close proximity to the mouth and avoided air sampling losses. The average expiration air velocity was 11.7m/s for coughing and 3.9m/s for speaking. Under the experimental setting, evaporation and condensation effects had negligible impact on the measured droplet size. The geometric mean diameter of droplets from coughing was 13.5μm and it was 16.0μm for speaking (counting 1–100). The estimated total number of droplets expelled ranged from 947 to 2085 per cough and 112–6720 for speaking. The estimated droplet concentrations for coughing ranged from 2.4 to 5.2cm−3 per cough and 0.004–0.223cm−3 for speaking.

Transport and Removal of Expiratory Droplets in Hospital Ward Environment

The transport and removal characteristics of expiratory droplets at different supply airflow rates and “coughing” orientations were investigated both numerically and experimentally in a three-bed hospital ward setting. A Lagrangian-based particle-tracking model with near-wall correction functions for turbulence was employed to simulate the fate of the expiratory droplets. The model was tested against experimental droplet dispersion data obtained in an experimental hospital ward using Interferometric Mie Imaging and a light-scattering aerosol spectrometer. The change in airflow supply rate had insignificant effect on the transport and deposition of very large droplets (initial sizes ≥ 87.5 μm) due to the dominance of gravitational settling on these behaviors. Smaller droplets (initial sizes ≤ 45 μm) exhibited certain airborne behaviors. The effect of thermal plumes from heat sources was observed only when the supply airflow was low and when the droplet size was small, as observed in the vertical mixing patterns of the droplets of various sizes. Larger droplets tended to settle lower and lateral dispersion of the droplets became weak at the low supply airflow rate. The deposition characteristics for different surfaces in the room are described. The heat plumes seemed to obstruct small droplets from being deposited onto heated surfaces. More deposition was predicted in the lateral injection case compared with the vertical injection case. Adopting near-wall correction for turbulence in the model reduced the predicted deposition removal fraction by 25% for 1.5 μm droplets. This reduction became less significant for larger droplets due to the smaller dependence on turbulent diffusion in their deposition.

Dispersion of Expiratory Droplets in a General Hospital Ward with Ceiling Mixing Type Mechanical Ventilation System

This study investigated the dispersion characteristics of polydispersed droplets in a general hospital ward equipped with ceiling-mixing type ventilation system. Injections of water test droplets containing non-volatile content were produced. The injections simulate human coughs with a similar droplet size distribution (peak size at 12 μ m) and airflow rate (0.4 L/s). The dispersion of test droplets was measured in-situ by interferometric Mie imaging (IMI) method combined with an aerosol spectrometer. A multiphase numerical model was employed to simulate the droplet dispersion tracks to provide additional transient position tracking data. Results show that the small size group of droplets or droplet nuclei (initial size ≤45 μ m) behaved airborne transmittable as some nuclei stayed airborne for more than 360 s. The dispersions were strongly affected by the ventilation airflow pattern. The expiratory droplets exhibited a two-stage lateral dispersion behavior, in which rapid dispersion was found in the early “initial dispersion” stage and then the dispersion became much slower in the subsequent “stable” stage. The exhaust air vents significantly enhanced lateral dispersions towards their direction. Droplets in the large size group (initial size = 87.5 μ m and 137.5 μ m) were subjected to heavy gravitational effect and stayed airborne for less than 30 s. Results indicate that the location of exhaust air vents has significant impact on the dispersion pattern of expiratory droplets. It should be carefully considered in designing ventilation systems for health-care settings.

Transport Characteristics of Expiratory Droplets and Droplet Nuclei in Indoor Environments With Different Ventilation Airflow Patterns

Expiratory droplets and droplet nuclei can be pathogen carriers for airborne diseases. Their transport characteristics were studied in detail in two idealized floor-supply-type ventilation flow patterns: Unidirectional-upward and single-side-floor, using a multiphase numerical model. The model was validated by running interferometric Mie imaging experiments using test droplets with nonvolatile content, which formed droplet nuclei, ultimately, in a class-100 clean-room chamber. By comparing the droplet dispersion and removal characteristics with data of two other ceiling-supply ventilation systems collected from a previous work, deviations from the perfectly mixed ventilation condition were found to exist in various cases to different extent. The unidirectional-upward system was found to be more efficient in removing the smallest droplet nuclei (formed from 1.5 mum droplets) by air extraction, but it became less effective for larger droplets and droplet nuclei. Instead, the single-side-floor system was shown to be more favorable in removing these large droplets and droplet nuclei. In the single-side-floor system, the lateral overall dispersion coefficients for the small droplets and nuclei (initial size </=45 mum) were about an order of magnitude higher than those in the unidirectional-upward system. It indicated that bulk lateral airflow transport in the single-side-floor system was much stronger than the lateral dispersion mechanism induced mainly by air turbulence in the unidirectional-upward system. The time required for the droplets and droplet nuclei to be transported to the exhaust vent or deposition surfaces for removal varied with different ventilation flow patterns. Possible underestimation of exposure level existed if the perfectly mixed condition was assumed. For example, the weak lateral dispersion in the unidirectional ventilation systems made expiratory droplets and droplet nuclei stay at close distance to the source leading to highly nonuniform spatial distributions. The distance between the source and susceptible patients became an additional concern in exposure analysis. Relative significance of the air-extraction removal mechanism was studied. This can have impact to the performance evaluation of filtration and disinfection systems installed in the indoor environment. These findings revealed the need for further development in a risk-assessment model incorporating the effect of different ventilation systems on distributing expiratory droplets and droplet nuclei nonuniformly in various indoor spaces, such as buildings, aircraft cabins, trains, etc.

Experimental Study of Dispersion and Deposition of Expiratory Aerosols in Aircraft Cabins and Impact on Infectious Disease Transmission

The dispersion and deposition characteristics of polydispersed expiratory aerosols were investigated in an aircraft cabin mockup to study the transmission of infectious diseases. The airflow was characterized by particle image velocimetry (PIV) measurements. Aerosol dispersion was measured by the Interferometric Mie Imaging (IMI) method combined with an aerosol spectrometer. Deposition was investigated using the fluorescent dye technique. Downward air currents were observed near the seats next to the side walls while upward airflows were observed near other seats. The downward airflow showed some effects on suppressing the dispersion of aerosols expelled by the passenger sitting in the window seat. Results show that the cough jet could bring significant amount of aerosols forward to the row of seats ahead of the cougher and the aerosols were then dispersed by the bulk air movements in the lateral direction. The aerosols expelled from a cough took 20–30 s to reach the breathing zones of the passengers seated within two rows from the cougher. Increasing the ventilation rate improved the dilution and reduced the aerosol exposure to passengers seated close to the source, but the aerosol dispersion increased, which heightened the exposure to passengers seated further away. 60–70% of expiratory aerosols in mass were deposited, with significant portions on surfaces close to the source, suggesting that disease transmission risk via indirect contact in addition to airborne risk is possible. The physical transport processes of expiratory aerosols could be used to shed insights on some epidemiological observations on in-flight transmission of certain infectious diseases.

Modeling the Fate of Expiratory Aerosols and the Associated Infection Risk in an Aircraft Cabin Environment

The transport and deposition of polydispersed expiratory aerosols in an aircraft cabin were simulated using a Lagrangian-based model validated by experiments conducted in an aircraft cabin mockup. Infection risk by inhalation was estimated using the aerosol dispersion data and a model was developed to estimate the risk of infection by contact. The environmental control system (ECS) in a cabin creates air circulation mainly in the lateral direction, making lateral dispersions of aerosols much faster than longitudinal dispersions. Aerosols with initial sizes under 28 μm in diameter can stay airborne for comparatively long periods and are favorable for airborne transport. Using influenza data as an example, the estimated risk of infection by inhalation are at least two orders of magnitude higher than the risk of infection by contact. An increase in the supply airflow rate enhances ventilation removal and the dispersion of these aerosols. It reduces the risk of infection by inhalation for passengers seated within one row and one column from the index patient but it increases the risk for passengers seated further away. The deposition fraction increases with aerosol size. The ECS supply airflow rate has insignificant impact on the deposition behavior of these large aerosols, making the impact on the risk of infection by contact insignificant. Comparatively, the contact behavior of passengers is highly influential to the contact infection risk. Passengers seated within one row from the index patient are subject to contact risks that are one to two orders of magnitude higher than are passengers seated further away.

Mathematical models for the van der Waals force and capillary force between a rough particle and surface.

The capability of predicting the adhesion forces between a rough particle and surface including the van der Waals force and capillary force is important for modeling various processes involving particle surface retention and resuspension. On the basis of the fractal theory describing the behavior of multiple roughness scales and the Gaussian roughness distribution, a set of mathematical models for the van der Waals force and capillary force is proposed. The proposed models provide the adhesion force predictions in good agreement with the existing experimental data and converge to the previous classical solutions of the adhesion forces between a smooth particle and surface as the roughness goes to zero. The influences of roughness for the combination of particle and surface, relative humidity (RH), contact angle, and Hurst exponent toward the adhesion forces are examined using the proposed models. The decline mode of the adhesion force with surface roughness and contact angle, as well as the increase mode with RH and the Hurst exponent are reasonably predicted by the proposed models. The comparison between the proposed models and those from the existing studies is also performed, which shows the similarities and differences between the proposed models and the existing models.

A methodology for estimating airborne virus exposures in indoor environments using the spatial distribution of expiratory aerosols and virus viability characteristics

UNLABELLED This study investigated the feasibility of using the spatial distribution of expiratory aerosols and the viability functions of airborne viruses to estimate exposures to airborne viruses in an indoor environment under imperfectly mixed condition. A method adopting this approach was tested in an air-conditioned hospital ward. Artificial coughs were produced by aerosolizing a simulated respiratory fluid containing a known concentration of benign bacteriophage. The bacteriophage exposures estimated on the basis of the spatial aerosol distributions and its viability function were in reasonable agreement with those measured directly by biological air sampling and culturing. The ventilation flow and coughing orientation were found to play significant roles in aerosol transport, leading to different spatial distribution patterns in bacteriophage exposure. Bacteriophage exposures decreased with lateral distance from the infector when the infector coughed vertically upward. In contrast, exposures were constant or even increased with distance in the case of lateral coughing. The possibility of incorporating the proposed exposure estimation into a dose-response model for infection risk assessment was discussed. The study has also demonstrated the potential application of viability functions of airborne viral pathogens in exposure assessment and infection risk analysis, which are often unavailable in literature for some important communicable diseases. PRACTICAL IMPLICATIONS The proposed method makes use of the viability function of the virus and the spatial distribution of the expiratory aerosols for virus exposure estimation. Spatial differences in aerosol distribution and its influences on virus exposure in an air space can be determined. Variations in infectious dose with carrier aerosol size could also be considered. The proposed method may serve as a tool for further investigation of ventilation design and infection control in clinical or other indoor environments.

Exposure and cancer risk toward cooking-generated ultrafine and coarse particles in Hong Kong homes

Particulate matters generated during cooking contain various carcinogens. These particles consist of both ultrafine particles (nanoparticle) and coarse particles. Exposure and risk-assessment studies often use particle mass concentration as dosimetry, which ignores the impact of ultrafine particles due to their insignificant mass compared to coarse particles. This study aims at characterizing the cancer risk toward cooking-generated particulate matter using Hong Kong homes as an example. A risk-assessment scheme modified from an existing risk model was developed to consider the cancer risk contributed from both fine ultrafine and coarse particles. Exposure assessment was conducted based on particle concentration data measured in 16 Hong Kong homes. The predicted cancer risk was then compared to the cancer incidence rate in Hong Kong to examine its appropriateness. It was found that the ultrafine particles contribute a much higher risk than that of coarse particles, and the modified risk-assessment scheme gives an estimate much closer to the incidence rate than the conventional scheme. Use of grease extractor cannot completely contain the particles, and a significant portion of particles can be transported from kitchens to other regions of the homes. The modified risk assessment scheme can serve as a tool in assessing environmental quality as well as setting up design and ventilation guidelines and exposure standards on particulate matters.

Ultrafine Particle Emissions from Cigarette Smouldering, Incense Burning, Vacuum Cleaner Motor Operation and Cooking

Combustion activities such as cigarette smouldering, incense burning and cooking are important sources of particulate matters (PM) in indoor environments. Vacuum cleaning contributes to the non-combustion-related sources of PMs. In this study, we investigated the rates at which ultrafine particles (UFPs) are emitted from cigarettes, incenses and vacuum cleaners in a small test chamber. UFP emission from cooking was obtained by conducting experiments in a residential kitchen. Particle number concentrations and size distributions from these sources were measured using a scanning mobility particle sizer (SMPS) and the UFP emission rates were then determined using a material balance approach. The mean UFP emission rates of cigarette smouldering and incense burning were found to be 3.36 ± 0.34 and 0.44 ± 0.33 × 1011 particles min−1 in terms of the number emission rate, or 22.78 ± 1.21 and 3.48 ± 2.98 × 1015 nm2 min−1 in terms of the surface area emission rate, respectively. Vacuum cleaner motor operation and cooking showed high variations in UFP emission, in the ranges 0.013–0.066 and 4.70–148.29 × 1011 particles min−1, respectively. A database of emission rates for UFP sources can be compiled, which will be useful in estimating the UFP concentration and subsequent human exposure.