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

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Experimental Study of Dispersion and Deposition of Expiratory Aerosols in Aircraft Cabins and Impact on Infectious Disease Transmission G. N. Sze To, M. P. Wan, C. Y. H. Chao, L. Fang & A. Melikov To cite this article: G. N. Sze To, M. P. Wan, C. Y. H. Chao, L. Fang & A. Melikov (2009) Experimental Study of Dispersion and Deposition of Expiratory Aerosols in Aircraft Cabins and Impact on Infectious Disease Transmission, Aerosol Science and Technology, 43:5, , DOI: / To link to this article: Published online: 26 Feb Submit your article to this journal Article views: 1007 Citing articles: 39 View citing articles Full Terms & Conditions of access and use can be found at

2 Aerosol Science and Technology, 43: , 2009 Copyright American Association for Aerosol Research ISSN: print / online DOI: / Experimental Study of Dispersion and Deposition of Expiratory Aerosols in Aircraft Cabins and Impact on Infectious Disease Transmission G. N. Sze To, 1 M. P. Wan, 1,2 C. Y. H. Chao, 1 L. Fang, 3 and A. Melikov 3 1 Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong 2 School of Mechanical Engineering, Kyungpook National University, Daegu, Korea 3 International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Denmark 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 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 % 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. Received 5 June 2008; accepted 7 January This research was financially supported by the Research Grants Council of the Government of the Hong Kong S.A.R. through grant number The authors would like to acknowledge the technical and partial financial support from the International Centre for Indoor Environment and Energy, Technical University of Denmark. Address correspondence to C. Y. H. Chao, Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. meyhchao@ust.hk 1. INTRODUCTION Air travel can play an important role in the worldwide spread of infectious diseases (Mangili and Gendreau 2005). A recent example is the transmission of Severe Acute Respiratory Syndrome (SARS) on the flight from Hong Kong to Beijing in March 2003 (Olsen et al. 2003). Twenty-two people were infected with SARS during the flight, which led to over 300 secondary cases. The World Health Organization s (WHO) global influenza preparedness states that air travel could hasten the spread of an emerging disease and decrease the time available for preparing interventions, making it difficult to halt the spread of a pandemic disease (WHO 2005). The transmission of diseases during and after air travel and its prevention are topics of great concern. Infection control during and after air travel is difficult. High occupant density and relatively long exposure times may increase the risk of disease transmission in aircraft cabins. The best practice would be to prevent the passengers with infectious disease from boarding the plane. However, it is practically prohibitive to assess the health of all passengers before their flight. It is also difficult to trace and contact passengers who might have been exposed to infectious diseases during air travel, since many diseases have incubation periods of several days or even weeks before the symptoms are observable and some passengers may continue to travel. The limited availability and accuracy of airline records also add difficulties in identifying these passengers (WHO 2006). Over the past decades, researchers and the aerospace industry made improvements to the comfort levels and hygiene in aircraft cabins (Space et al. 1999). Many studies suggested that the risk of infection during air travel was related to the environment in the aircraft cabin (e.g., Spengler and Wilson 2003; Mangili and Gendreau 2005). A better design of the aircraft cabin environment control system (ECS) would lead to better control of infectious disease transmission during air travel. In most modern commercial aircraft, conditioned air is supplied 466

3 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 467 from overhead supply air slots. The air is circulated in the cabin and then exits from the air return outlets located along both side walls of the cabin near the floor. Air enters and leaves the cabin at approximately the same row of seats. Airflow from front to back of the cabin (longitudinal) is minimal. This air circulation pattern divides the air flow into sections to limit the spread of airborne particles throughout the passenger cabin (Mangili and Gendreau 2005). Most commercial aircraft cabins have air changes per hour during cruising (Hunt and Space 1995) and fewer during descent and on the ground (WHO 2006). Commercial aircrafts generally cruise at altitudes of around 11,000 to 12,200 m, where the atmospheric air has low humidity, low temperature and low pressure. After the atmospheric air is compressed and heated, its relative humidity (RH) becomes only about 5% or less (Space et al. 1999; García Río et al. 2007). To increase the relative humidity and energy efficiency, most commercial aircrafts recirculate approximately 50% of the cabin air. With recirculation, the in-flight relative humidity in commercial aircraft cabins ranges from 2% to 38% (Nagda et al. 1992). Recirculated cabin air is filtered by high efficiency particulate air (HEPA) filters, which can remove over 99.9% of airborne bacteria and viruses (Hunt et al. 1995). However, even with those controls, in-flight infection transmission is still reported from time to time. Further improvements are needed and understanding of the fundamental transmission mechanisms is obviously lacking. Many reported infections transmitted in-flight are associated with respiratory diseases, for example, Tuberculosis (TB), SARS, influenza, measles and smallpox (Mangili and Gendreau 2005). Airborne and contact transmissions are of greater concern regarding the spread of disease during air travel compared to other means of transmission. There were many studies that investigated the transmission of diseases on commercial airliners, including epidemiology studies that analyzed the outbreak of diseases during flight, exposure assessment of bioaerosols on-board (e.g., Nagda et al. 1992), and others studies which investigated the airflow and the dispersion of expiratory aerosols in aircraft cabins. To study the dispersion of expiratory aerosols in aircraft cabins and its interaction with ventilation airflows, numerical simulations and experimental studies have been conducted (e.g., Rydock 2004; Lin et al. 2005; Wang et al. 2008; Zhang and Chen 2007a, b). However, most of these experimental studies employed gaseous phase surrogates or monodispersed particles to represent the pathogenic airborne particles. Airborne pathogens are in aerosol form by themselves and pathogens of many known airborne transmittable diseases are usually dispersed by attaching to airborne carriers such as human expiratory aerosols, which are in droplet form and polydispersed (Nicas et al. 2005). Studies have shown that aerosol dispersion (e.g., Chao et al. 2008; Wan et al. 2007) and deposition mechanisms (e.g., Bouilly et al. 2005; Thatcher et al. 2002; Lai and Nazaroff 2005, Byrne et al. 1995) are size dependent. To reveal the dispersion characteristics of expiratory aerosols more realistically, the size effect should be considered. However, comprehensive experimental investigations of the transmission of pathogenic particles in aircraft cabins are lacking. This study focused on investigating the dispersion and deposition characteristics of expiratory aerosols in an aircraft cabin mockup as well as to identify the effects of ventilation on the dilution and removal of the expiratory aerosols. Based on this investigation with analysis on some epidemiological observations available in the literature, it aimed to find out whether there are linkages between the physical transport processes of the expiratory aerosols and the spread of infectious disease via the airborne and indirect contact routes. 2. METHODS 2.1. Facility, Equipment, and Materials Experiments were conducted in an aircraft cabin mockup facility located in the International Centre for Indoor Environment and Energy, Technical University of Denmark. The outer shell of the mockup cabin was mainly made from aluminum and the entire mockup was built inside another climate chamber such that the internal wall surface temperature of the cabin could be controlled. The cabin had internal dimensions of 4.9 m 3.2 m 2.0 m (W L H). More detailed dimensions are shown in Figure 1a and b. Inside the cabin were 21 seats, arranged in three rows of seven seats with two aisles. Supply air was distributed through two overhead slots located at the middle part of the ceiling and the supply air jets were directed towards the two side walls of the cabin. The supply air was 100% fresh air and there was no air recirculation to prevent recirculation of aerosols introduced during the experiments. This is justified by the fact that most modern aircrafts are fitted with HEPA filters in the recirculation system (Wang et al. 2006), providing at least 99.97% filtration efficiency for particles. The condition of the supply air was controlled by an independent air-conditioning system with a rotary desiccant dehumidifier. The maximum airflow supply rate in the cabin was 200 L/s with a control accuracy of ±2% of the actual flow. This corresponds to supplying 9.5 L/s of air per person. This is close to the value of 9.7 L/s of a popular modern commercial aircraft (Pierce et al. 1999). The cabin air temperature was controlled by monitoring the conditions of the air exhausted from the cabin. The control accuracy of air temperature was ±0.2 C and RH could be controlled down to 2% at 23 C with a stability of ±2%. The cabin air was exhausted through two cylindrical ducts running along the entire length of the mockup. A duct was installed along the lower edge of the floor on each side. The duct walls were perforated for air to pass through. More detailed description of the mockup can be found in Størm-Tejsen et al. (2007). During the experiments, the cabin was maintained at a positive pressure of about 7 Pa relative to the surroundings to prevent infiltration of background aerosols. The cabin mockup was built in a laboratory on ground level, meaning that the experiments were run at atmospheric pressure. This could be different from the pressure conditions in actual aircraft cabins. Fifteen heating cylinders (60 W each)

4 468 G. N. SZE TO ET AL. were placed at the seats as passenger manikins. Not all of the seats were filled up with heating cylinders because some seats were used to locate other instrumentation accessory and power board equipment. Figure 1b shows the detailed configuration. The cylinders and the instruments produced a heat density of around 60 W/m 2 of floor area. When a cylinder was placed on a seat, the top of the cylinder was about 0.2 m higher than the top of the seat back. However, it should be noted that the heating cylinders had no legs. This might lead to some differences to the real situation. An in-house-built droplet generator was placed in one of the seats to simulate a coughing passenger. The generator produced polydispersed test droplets with proper non-volatile content to represent the pathogenic airborne particles. Droplets were FIG. 1. (a) Schematic diagram of the cabin mockup side view and back view, dimensions in mm (not to scale). (b) Schematic diagram of the cabin mockup top view (not to scale). (Continued)

5 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 469 FIG. 1. (Continued) injected horizontally forward at the mouth position of a sedentary passenger by a puff release of compressed air to mimic a cough. The droplet generator consisted of a pneumatic nozzle and gas and liquid supply lines together with pressure and flow rate control systems. Compressed air was used as the gas supply and a simulated saliva solution was used to feed the liquid supply line. The compressed air and liquid solution were externally mixed at the tip of the nozzle through an air cap, producing a full-cone injection spray. The simulated saliva solution, which was used to produce the droplets, consisted of 76 g of glycerin and 12 g of salt (NaCl) in 1 liter of distilled water to simulate the non-volatile content in respiratory fluid. With this composition, the non-volatile content occupied about 6% of the volume of the droplets after complete desiccation, corresponding to about 39% of the original droplet diameter. The volume weighting between the non-volatile content and the water content in the solution used for droplet generation was similar to the actual weighting in human respiratory fluid reported by Effros et al. (2002). In

6 470 G. N. SZE TO ET AL. FIG. 2. (a) Number and volume-weighted droplet size distribution profile measured at 5 mm downstream from the droplet generator outlet. (b) Velocity field measured at 5 mm downstream from the nozzle outlet of the droplet generator during injection.

7 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 471 each one-second injection, ml of simulated saliva solution were aerosolized and introduced into the cabin with 0.4 liter of air. A detailed description of the droplet generator can be found in Wan et al. (2007). The size distribution of the droplets produced by the droplet generator was characterized using the Interferometric Mie Imaging (IMI) technique (Wan et al. 2007). The IMI system measured droplets greater than 2 µm in diameter. The characterization was done inside the cabin mockup with the air temperature and humidity maintained at 24 C and 5% RH, respectively. Figure 2a shows the size distribution profile of the generated droplets measured at about 5 mm from the tip of the outlet in terms of df n /dlnd p, where f n is the number fraction and D p is the droplet size. A log-normal curve was fitted to the profile reported by Duguid (1946) for coughing and the curve is shown in the figure for comparison. Volume-weighted droplet size profile is also shown. The initial velocity of the air jet produced by the droplet generator was also characterized by particle image velocimetry (PIV) measurements and the result is shown in Figure 2b. The initial velocity of the injection air jet was about 10.6 m/s, which was similar to the initial velocity of a human cough air jet as reported by Zhu et al. (2006). This was to ensure that the generated droplets had a similar initial momentum compared to the momentum of droplets generated by actual coughing. With this injection air velocity, the droplets took less than 0.5 ms to travel from the tip of the generator outlet to the IMI measurement position. For droplets of larger than 2 µm in size, it is expected that droplet shrinkage due to evaporation was negligible within such short time. The results shown in Figure 2a is considered as the size distribution before evaporation. PIV measurements conducted at different horizontal distances from the injection outlet showed that at about 1 m, the velocity of the cough jet became indistinguishable from the background airflows in the cabin, suggesting that the cough jet had a characteristic momentum length of about 1 m. However, the droplets might not have the same momentum length, especially for the larger droplets Characterization of the Airflow Pattern The airflow pattern at selected locations in the cabin was characterized by PIV measurements. The PIV measurement locations were distributed in the center cross sectional plane (at y = 1.6 m) cutting through row B, as shown in Figure 3. Each rectangle in the figure represents a planar PIV measurement field of mm. Some measurements were made directly above the heating cylinders and a measurement was made immediately at the supply air outlet. Flow seeding was achieved by atomizing glycerin solution (75 wt% distilled water; 25 wt% pure glycerin (Acros Organics)) using a four-nozzle atomizer (LaVision, Aerosol Generator), which generated seeds with a mean diameter of 0.25 µm. The illumination laser source was a double-pulsed Nd: YAG laser of 532 nm wavelength with the highest repetition frequency of 15 Hz (New Wave Research, Solo II). Images of the illuminated flow field were captured by a dual-frame CCD camera with a resolution of pixels (LaVision, Imager Intense) mounted with a 532 nm optical filter. The dual-frame images were taken at 5Hz during the measurements. The captured images were processed by an integrated data acquisition and post-processing software (LaVision, DaVis 7.0). Measurements were conducted under two supply airflow rates: 100 L/s and 200 L/s, which corresponded to 4.8 L/s/person and 9.5 L/s/person, respectively, under full capacity. These flow rates were also used in the aerosol dispersion experiments Measurement of Aerosol Dispersion The dispersion pattern of the expiratory aerosols in the cabin was measured by an aerosol spectrometer (GRIMM, model 1.108) combined with the IMI system. The aerosol spectrometer measured aerosols in 16 different size bins ranging from 0.3 µm to20µm and the IMI system was used to measure aerosols larger than 20 µm in size. The sampling flow rate of the aerosol spectrometer was 1.2 L/min and the averaging time for each data point was 1 s. A simple estimation of the Stokes numbers at the sampling probe of the aerosol spectrometer (Hinds 1999) indicated that air sampling could be assumed to be isokinetic. Since the amount of suspended aerosols reduced considerably with time, especially for the large aerosols, the aerosol concentrations from 60 s onwards were taken as the average aerosol concentrations of every 10 data points during data processing, i.e., 10 s, to improve detection resolution. The aerosol measurements were conducted in a grid of measurement points as shown in Figure 1. Before the droplet injection, the background aerosol level in the cabin was measured by the aerosol spectrometer for 1 minute, which would later be subtracted from the aerosol counting results obtained after the injection. A one-second injection of droplets was then produced. The aerosol measurement instruments measured the aerosol number counts every second at one of the measurement points for 6 minutes after each injection. The instruments were then moved to another measurement point and the above process was repeated until all the measurement locations were covered. Since the injection and measurement procedures were repeated until all the measurement points were covered, the repeatability of the measurement was checked at selected points. Figure 4 shows the repeatability of two measurement runs at the breathing height (1.1 m) of seat B4 and A4 under the 100 L/s supply airflow rate condition. The checking measurements show satisfactory repeatability. The aerosol concentrations shown in Figure 4 (also in the later aerosol concentration plots, i.e., Figures 5, 8, and 9) have already had the background concentration offset. Under each experimental condition, dispersion measurements were conducted at two different injection points, middle and side. The middle injection point simulated a passenger seated at the middle seat of the last row (seat C4) while the side injection point simulated a passenger seated next to the side wall in the same row (seat C7). These two locations were

8 472 G. N. SZE TO ET AL. FIG. 3. PIV measurement results at selected locations under the 200L/s supply airflow rate and the anticipated air circulation pattern in the cabin. FIG. 4. Repeatability test at the breathing level (1.1 m) of seat B4 and A4 under the 100 L/s, middle injection condition.

9 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 473 FIG. 5. (a) Total aerosol concentration profiles at the breathing level at selected measurement locations under the 100L/s supply airflow rate, 5% RH, middle injection. (b) Total aerosol concentration profiles at the breathing level at selected measurement locations under the 100L/s supply airflow rate, 5% RH, side injection. found to be subjected to different airflow currents during the airflow characterization, as discussed below. The inlet air temperature was maintained at 24 C in all the experiments. All measurements were performed under a RH of 5% to simulate the cruising condition. The configuration of the locations of the heating cylinders was the same for experiments of each injection location (see Figure 1b). It should be noted that evaporation of the water content from the droplets occurred after the droplets were introduced into the air. By the time that the droplets were measured, their sizes had shrunk. The aerosol dispersion results presented below are based on this measured aerosol size Measurement of Aerosol Deposition in the Cabin The deposition of expiratory aerosols in the aircraft cabin was measured using a fluorescence dye technique similar to that employed in numerous studies on particle deposition mechanisms (e.g., Thatcher et al. 1996; Lai and Nazaroff 2005). For this set of measurements, water-soluble fluorescent dye (Kingscote Chemicals, Bright Dyes FLT Yellow/Green) was added to the simulated saliva solution (2 g of dye per liter of the solution). Before the injection of test droplets, all the surfaces of the seats, floor and walls in the cabin were covered with sheets of polyethylene film (Witre, Gennemsigtig film,

10 474 G. N. SZE TO ET AL. 15 µm thick). The entire surface (except the bottom) of each heating cylinder was covered with aluminum foil. The polyethylene film and the aluminum foil were mounted tightly on the surfaces so that there was no substantial change in the shape of the objects. It is expected that there was no major effect on the airflow due to the mounting of the film/foil. All the surfaces in the cabin were covered except that, due to the position of the droplet generator, deposition on the seat surface of the injection point could not be measured. After mounting the polyethylene film and the aluminum foil onto the surfaces, the unmanned cabin was kept operating for 1 day before conducting the deposition experiment. This was to ensure that the charge potential induced during the mounting process would have essentially decayed before the deposition experiment was conducted. A study showed that the externally induced charge potential on polyethylene material would substantially decay in a few minutes (Xu et al. 2007). However, this exercise did not completely eliminate the electrostatic effect but only avoid unusual charge potential induced by the film mounting process. Two deposition experiments were run, the middle and the side injection experiment. Measurements of deposition were conducted at 24 C, 5% RH and 200 L/s supply airflow rate. The configurations of heating cylinder and droplet injector locations for the middle and the side injection deposition experiments were the same as that in the dispersion experiments described in the previous section. Five rounds of one-second droplet injections were done in each deposition experiment. There was a one-hour time gap between each injection to ensure that the aerosols produced in the previous injection were completely removed from the air before continuing with another round of injection. After finishing all five rounds of droplet injection in an experiment, the polyethylene films and aluminum foil containing the deposited fluorescent aerosols were then carefully detached and replaced with fresh polyethylene films and aluminum foil for another experiment. The fluorescent aerosols deposited on each surface were extracted by soaking the entire piece of film/foil covering that surface into a known volume of distilled water. A sample of the water containing the extracted fluorescent dye was then analyzed in a spectrophotometer (Hach, DR/2500), which determined the concentration of the dye in that sample by measuring the light absorption of the sample using a light source at the wavelength of maximum absorbance of the fluorescent dye (490 nm). The readings of the spectrometer were correlated to the concentrations of fluorescent dye by calibrating with a set of standards with known amounts of fluorescent dye in distilled water. The lower detection limit of the spectrophotometer to the fluorescent dye was about 0.2 µg. The amount of dye deposited on that film/foil was then calculated from the measured amount of dye in the water sample. Both injection locations were considered. 3. RESULTS 3.1. The Airflow Pattern in the Cabin Airflow velocity profiles at selected locations for the 200 L/s case are shown in Figure 3. Under the current setting, the initial velocity of the supply jet was about 3 m/s. The supply jet projected towards the side. Downward air currents of about m/s were created in the head region of seat 7. At the next seat (seat 6), upward air currents of about m/s were observed. PIV measurements made in longitudinal planes at the head region of seat 4 found mainly upward air currents of around m/s. No significant longitudinal airflow was observed. This suggests that the supply air jet attached to the walls and created downward air currents only at seat 7. The supply air jet had a direct effect to the airflow at seat 7 (the seat next to the side wall) which created downward air currents in the head region. The upward airflows observed in the head region of other seats might be induced by the ventilation airflows and might be assisted by the heat plumes of the cylinders. Air currents at the leg level were found to be flowing towards the center of the cabin. The PIV measurement results at different locations indicate general lateral airflow pattern in the cabin to follow the bold arrows shown in the figure. This airflow pattern was similar to patterns reported in other experimental or numerical studies in aircraft cabins (e.g., Zhang and Chen 2007a; Wang et al. 2008). The airflow circulation pattern was found to be similar in the 100 L/s condition but the airflows were at lower velocities. The initial velocity of the supply jet was about 1.5 m/s and the downward air currents in the head region of seat 7 were reduced to about m/s in the 100 L/s case. The velocities of upward air currents at other seats were also lower than those measured under the supply airflow rate of 200 L/s. It is worth of notice that the airflow pattern described above does not necessarily apply to all types of commercial aircrafts. The cabin mockup used in the current study had a 2-aisle configuration with only 3 rows of seats. The lack of longitudinal flow found in the mockup may not resemble the longitudinal flow condition in real cabins, which are much longer than the current cabin mockup. There was also no gaspers (the small adjustable air nozzles found in some aircrafts) in the mockup. Therefore, the current results cannot reflect the effects of these small air jets on the cabin airflow Aerosol Dispersion Pattern and Concentration Change Effect of Cougher Location From the aerosol counting results, the change in the aerosol concentration at selected measurement locations for the supply airflow rate of 100 L/s are plotted in Figure 5. The sudden increase in the aerosol concentration at the beginning of each curve indicates that the aerosols had reached that measurement location. The time taken for this sudden increase in the aerosol concentration to occur indicates the time required for

11 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 475 the aerosols to be dispersed from the injection point to that location. The injection jet moved many of the aerosols to the seat in front of the injection location (B4 for the middle injection case; B7 for the side injection case) immediately after the injection. For the middle injection case, Figure 5a shows that the aerosols took around 10 s to be dispersed to the farthest measurement location in the same row (seat C7). Referring back to Figure 3, the air circulation pattern in the two halves of the cabin was symmetric. It stands to reason that the aerosol dispersion pattern was also symmetric in the middle injection case. The expiratory aerosols from a passenger sitting in a middle seat could cover the entire width of the cabin in about 10 s. In the side injection case, Figure 5b shows that the aerosols took around 30 s to reach seat C4, located at the same distance from the injection point as seat C7 in the middle injection case. A similar trend can also be observed for 200 L/s supply airflow rate cases. However, the aerosols covered only half of the width of the cabin by reaching seat C4 since the injection was produced on one side. The aerosols took about 60 s to reach the other side of the cabin (seat C1) in the side injection case, compared to about 10 s in the middle injection case. Referring to the general airflow pattern shown in Figure 3, the airflow pattern in the cabin could be characterized into three virtual zones, two air circulation zones at the two sides and a mixing zone at the center. It can be inferred that the aerosols released in the middle injection case were mixed with the uprising air in the center mixing zone and then distributed to the sides. In contrast, in the side injection case, the downward airflow current at seat 7 pushed down many aerosols after the injection such that the aerosols had to pass through obstructions (seats and manikins) in order to reach the center zone of the cabin. As a result, lateral dispersion in the middle injection case was quicker than in the side injection case. In addition, many aerosols were deposited as they passed through the obstructions and, to a certain extent, some aerosols were removed by ventilation. Dispersion of aerosols to the half cabin further away from the injection point was hindered in the side injection case. Measurements made in the further half of the cabin in the side injection case showed that the aerosols took more than 50 s to reach seat C1 at the 100 L/s supply airflow rate. The peak aerosol concentration found at C1 was less than 5% of that at C4. In contrast, in the middle injection case, many aerosols were lifted up by the upward airflow current along seat 4 after the injection. The high-velocity airflows from the supply slots near the ceiling then pushed the aerosols laterally towards the side of each half of the cabin across the space above the seats. This dispersion pattern greatly enhanced the mixing of aerosols compared to the side injection case. To give an overview of the dispersion patterns of the aerosols, the time evolution of the total aerosol concentration contours of the XY-plane at the passenger breathing height (1.1 m) were plotted in Figure 6. The contours show that the aerosols were pushed along the injection direction and reached one row of seats in front of the injection point by the cough jet. Given the air circulation pattern in the cabin, it can be observed that, in the middle injection cases (Figure 6a), the aerosols dispersed quicker in the lateral direction than in the longitudinal direction. Excluding the effect of the injection jet, which immediately brought many aerosols to row B, at the 100 L/s supply airflow rate, the aerosols took about 10 s to reach the measurement point at A4, which was 0.8 m away from B4 in the longitudinal direction. The aerosols also took about 10 s to reach the measurement point at C7, which was 2.4 m away in the lateral direction. Similar observations could be also found with the 200 L/s supply airflow rate. However, this trend was not as obvious in the side injection cases, as observed in Figure 6b. This may be due to the suppression effect by the downward airflow near the side wall, which slowed down the lateral dispersion compared to the middle injection cases Effect of the Supply Airflow Rate Figure 6 shows that it took about 20 s for the aerosols to reach all the measurement points at the breathing level at the supply airflow rate of 200 L/s for both injection locations. At 100 L/s, it took more than 30 s. This suggests that the dispersion was enhanced with the higher supply airflow rate. It is also observed that under a higher supply airflow rate, the influence of the bulk air circulation pattern on the dispersion of the aerosols became more significant. The side injection YZ plane concentration contours are shown in Figure 7. The YZ contours were plotted using the data obtained at the plane cutting across the injection location at seat 7. The injected droplets were pushed down by the downward airflow near the side wall of the cabin 10 s after injection. However, these effects were not as strong under a lower supply airflow rate. This suggests that the airflow pattern can have significant influence on the dispersion of expiratory aerosols under sufficient bulk air velocity. Figure 7 shows that the longitudinal dispersion at the breathing level was obstructed by the seats such that the aerosols could only disperse to other rows of seats through the space above and below the seat. To analyze the effect of the airflow rate on the exposure of expiratory aerosols by different passengers, the exposures of six passengers seated at different distances from the injection point were compared under the two supply airflow rates. Their exposures were estimated by the total volume of droplets measured at the breathing height in a six-minute period after the injection multiplied by the pulmonary ventilation rate. A similar estimation method was used in a previous study (Sze To et al. 2008) for pathogen exposure but, here, the pathogen survivability function was excluded. The exposure is to Exposure = p 0 n c(t) i v i dt, [1] where p is the pulmonary ventilation rate (taken as 7.5 L/minute), t o is the time interval (6 minutes), n is the total i=1

12 476 G. N. SZE TO ET AL. FIG. 6. (a) XY-contours of total aerosol concentrations (5% RH, z = 1.1 m, middle injection). (b) XY-contours of total aerosol concentrations (5% RH, z = 1.1 m, side injection). number of droplet size bins, c(t)i is the aerosol concentration of the i th size bin at time t [#/Lair ], vi is the mean droplet volume [m3 ] of the i th size bin at the measurement point. The estimated total exposures, E, are shown in Table 1. The estimated exposures to aerosols smaller than 5 µm in size, e, are shown in Table 2. This size range corresponds to the droplet nuclei, the major carrier for airborne infection transmission, indicated in Sehulster et al. (2004). The distribution patterns for E and e are essentially similar. This may be because the aerosols smaller than 5 µm in size was a major component (accounted more than 50% in most seats) of the total exposure (can be seen by comparing Tables 1 and 2). Table 1 shows that, regardless of the supply airflow rate and injection location, E decreased as the distance from the injection point increased. The passenger seated immediately in front of the injection point had the highest E, and the passenger seated next to the injection point also had fairly high E. The effect of supply airflow rate on E and e is shown by the ratios of E200 /E100 or e200 /e100 in the tables. Increasing the

13 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 477 FIG. 7. YZ-contours of total aerosol concentrations for side injection (5% RH, x = 4.52 m). supply airflow rate from 100 to 200 L/s had different impact on the exposures at different locations. At the seat immediately in front of the injection point, E reduced by about 20 30% and e reduced by about 20 80%, with the increase in supply airflow rate. This is probably due to the enhanced dilution effect with a higher supply airflow rate. In contrast, passengers seated further from the injection point (more than one row and one column of seats away) had higher exposures under a higher supply airflow rate. For instance, the exposure of the passenger seated at A7 (both E and e) nearly tripled at 200 L/s compared to 100 L/s. It suggests that the enhanced aerosol dispersion led to higher exposures at seats further away from the injection point despite the fact that dilution was also enhanced by the higher supply airflow rate. When the exposures of all seats were averaged, the E 200,avg /E 100,avg ratio was 0.85 for middle injection and 0.70 for side injection while the e 200,avg /e 100,avg ratio was 0.91 for middle injection and 0.60 for side injection. This data suggest that more significant reduction in exposure by increasing the supply airflow rate was observed in the side injection case. This could be because the stronger downward airflow near the side wall pushed more aerosols towards the exhaust duct under the higher supply airflow rate and enhanced their removal by extraction, as discussed in section Since the airflow in the cabin was divided into three zones, it might lead to the fact that very few aerosols were dispersed to the further half of the cabin, as evidenced by the very low exposures in seat A1 and C1. However, as only small amounts of aerosols were measured in the further half of the cabin, the measurement results in seat A1 and C1 would be influenced by measurement error to a great extent. This can be realized by estimating the uncertainty in the exposure estimation. A major source of uncertainty came from the resolution of the aerosol spectrometer. The instrument drew air sample at a rate of 0.02 L/s. Estimating from the sampling intervals used (1 s interval for the 1st minute and 10 s interval for the 2nd 6th minute), the total uncertainty in exposure caused by ±1 count of aerosol in every sample was ± ml. This is the maximum prediction by assuming there was ±1 count in every size channel and in every sample. However, aerosols larger than 15 µm in size were rarely captured in the experiments. If only the aerosols of 15 µm were concerned, the uncertainty in exposure was ± ml. These uncertainty levels were higher than the estimated exposures at seats A1 and C1. For Table 2, the uncertainty in estimated exposures for aerosols 5 µm in size was ml, comparable to the estimated exposures at seats A1 and C1. Another possible source of uncertainty was that the liquid flow meter in the droplet generator had a resolution of ±0.2mL/min(±0.003 ml/s). Compared to the total volume of droplet that was injected in every injection, the uncertainty was ±4%.

14 478 G. N. SZE TO ET AL. TABLE 1 Estimated exposures to expiratory aerosols at the passengers breathing level. Exposure unit in 10 5 ml Middle injection Seat A4 A5 A6 A7 100 L/s L/s Ratio E 200 /E Seat B4 B5 B6 B7 100 L/s L/s Ratio E 200 /E Seat C4 C5 C6 C7 100 L/s Inj. point L/s Ratio E 200 /E Side injection Seat A4 A5 A6 A7 A1 100 L/s L/s Ratio E 200 /E Seat B4 B5 B6 B7 100 L/s L/s Ratio E 200 /E Seat C4 C5 C6 C7 C1 100 L/s Inj. point L/s Ratio E 200 /E Measurements made in the further half of the cabin. E 100 Exposure in the 100 L/s case, E 200 Exposure in the 200 L/s case Effect of Aerosol Size Figures 8 and 9 show the aerosol concentration profiles at selected measurement points for different size ranges in the middle injection case. Droplet residues with sizes larger than 15 µm (above 37.5 µm in initial size according to the nonvolatile content of the test droplets) were found only in very small amounts in the measurements, suggesting that the dispersion of such large aerosols was limited in the cabin. Aerosols smaller than 7.5 µm indiameterswerewidelydispersedinthe cabin but only very limited amount of aerosols with diameters greater than 7.5 µm reached Row A. In Figure 8, aerosols with µm diameters reached the space at seat A4 in around 8 s, while aerosols with µm diameters took around 15 s to reach there. In Figure 9, aerosols with µm diameters could reach seat A4 in around 10 s, but aerosols with µm diameters took more than 20 s to reach that location. Removal of the larger aerosols was also faster. It can be observed from Figures 8 and 9 that no aerosols µm insizewere found after 150 s but many aerosols with smaller sizes could TABLE 2 Estimated exposures to expiratory aerosols with sizes 5 µm at the passengers breathing level. Exposure unit in 10 5 ml Middle injection Seat A4 A5 A6 A7 100 L/s L/s Ratio e 200 /e Seat B4 B5 B6 B7 100 L/s L/s Ratio e 200 /e Seat C4 C5 C6 C7 100 L/s Inj. point L/s Ratio e 200 /e Side injection Seat A4 A5 A6 A7 A1 100 L/s L/s Ratio e 200 /e Seat B4 B5 B6 B7 100 L/s L/s Ratio e 200 /e Seat C4 C5 C6 C7 C1 100 L/s Inj. point L/s Ratio e 200 /e Measurements made in the further half of the cabin. e 100 Exposure in the 100 L/s case, e 200 Exposure in the 200 L/s case still be found. Aerosols larger than 7.5 µm in size were found to be dispersed further in the longitudinal direction under the 200 L/s supply airflow rate when compared to the 100 L/s supply airflow rate Deposition of Expiratory Aerosols on Different Surfaces Figure 10 shows the percentage of dye mass deposited on different surfaces with reference to the total dye mass injected in each cough. The results are shown for five different types of surfaces. The definition and coverage of each type of surface are provided in Table 3. Figure 10 shows that the seat back which was immediately in front of the injection point had the largest amount of deposition in terms of mass among all surfaces. This could probably be due to the direct impaction of the injection jet to the seat back. The high inertia made these large aerosols favorable to impaction and deposition. Droplets with residue sizes larger than 15 µm accounted for over 90% in mass of the total injected droplets. The figure also shows that the majority of

15 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 479 the dye mass (over 50%) was deposited on surfaces within one row and one column of seat from the injection point. This might be due to the fact that the cough air jet (having a characteristic length scale of about 1 m) moved a portion of the test droplets a distance about one row in front of the injection point and then lost momentum. Some of these aerosols, especially the larger ones, settled quickly due to gravity. As a result, a significant amount of aerosols was deposited on the floor area about one row in front of the injection point, but not further. Less deposition was found on the floor area around the injection point in the middle injection case when compared to the side injection case. This might be caused by the downward airflow current near the side wall pushing down the expiratory aerosols, resulting in more deposition on the floor area around the injection point. The total deposited dye mass with reference to the total injected dye mass were about 70% and 60% in the middle injection and side injection cases, respectively. Since the deposition on the seat where the droplet generator was placed could not be measured, the actual percentage of deposition should be higher than these values. Considering the dye as the pathogens encased in expiratory aerosols, with such high portion of dye deposited, it suggests that indirect contact with surfaces contaminated by pathogen-laden expiratory aerosols may be another possible route of disease transmission in addition to the airborne route in aircraft cabins. Surfaces at close proximity and in front of the source are at higher risk of harboring infectious particles. Slightly less deposition occurred in the side injection case, which might be because the air exhaust ducts were located FIG. 8. (a) Concentration profiles for µm aerosols at the breathing level of selected seats under the 100 L/s supply airflow rate, 5% RH, middle injection condition. (b) Concentration profiles for µm aerosols at the breathing level of selected seats under the 100 L/s supply airflow rate, 5% RH, middle injection condition. (c) Concentration profiles for µm aerosols at the breathing level of selected seats under the 100 L/s supply airflow rate, 5% RH, middle injection condition. (d) Concentration profiles for µm aerosols at the breathing level of selected seats under the 100 L/s supply airflow rate, 5% RH, middle injection condition. (Continued)

16 480 G. N. SZE TO ET AL. FIG. 8. (Continued) at the bottom of the cabin side wall below the side injection point. Significant amounts of aerosols were extracted through the ducts after the injection under the influence of the downward airflow at seat 7. The deposition measurement was subjected to several sources of uncertainty. In preparing the simulated saliva solution with dye, the uncertainties in the mass and volume measurement instruments led to an uncertainty in dye concentration of ±3%. As estimated in Section 3.2.2, the droplet generator led to another ±4% uncertainty in the total mass of dye in each injection. In measuring the concentration of dye in the extraction solution using the spectrophotometer, the measurement uncertainty was estimated as ±0.3 µg of dye mass. There were also sources of uncertainty that could not be easily quantified, e.g., the loss of deposited dye during the detachment of the film/foil or during the handling of the film/foil in the laboratory for dye analysis. In a normal aircraft cabin, many surfaces are covered by fabric materials, including the seats, the clothing of passengers and the carpet of the floor area. However, in the current deposition experiments, surfaces in the cabin mock-up were covered with polyethylene film or aluminum foil to facilitate dye extraction. This may have some impact on the accuracy of the deposition measurements. The effects of surface materials on the deposition results were roughly estimated using the deposition constants of the testing materials (metal and polymer) and the actual materials (fabric) reported in the literature (experimental studies by Abadie et al and Wu et al. 2006). The ratio of deposition loss (actual deposition loss/measured deposition loss) for the manikin surface was roughly estimated as 1.6. The estimated ratio of deposition loss for the seat surface

17 DISPERSION AND DEPOSITION OF EXPIRATORY AEROSOLS IN AIRCRAFT CABINS 481 was 0.8. Readers should notice that the deposition constants taken from the literature were obtained under measurement conditions different from those in the current study. Others factors affecting the deposition of aerosols including the impaction effect of the cough jet, turbulence of the air, flow rate, electrostatic effect, etc., were not considered in this estimation. The above rough estimation simply provides an idea about how the surface material can affect the deposition measurement results. 4. DISCUSSION Three processes regarding the effect of different supply airflow rates on the spread of expiratory aerosols during air travel were observed: (1) the removal of expiratory aerosols is enhanced by dilution when the supply airflow rate is increased; (2) the influence of the airflow on the dispersion of expiratory aerosols is stronger under a higher supply airflow rate and; (3) expiratory aerosols disperse further with a higher supply airflow rate in the cabin. These processes suggest that an increase in the supply airflow rate has both positive and negative effects on infection control. Considering process 1, an increase in the supply airflow rate will generally reduce aerosol concentrations, hence the exposure, due to the enhanced dilution effect. However, considering process 3, the enhanced dispersion may lead to an increase in the number of passengers who are exposed to the aerosols. Whether this will favor the transmission FIG. 9. (a) Concentration profiles for µm at the breathing level of selected seats under the 200 L/s supply airflow rate, 5% RH, middle injection condition. (b) Concentration profiles for µm at the breathing level of selected seats under the 200 L/s supply airflow rate, 5% RH, middle injection condition. (c) Concentration profiles for m at the breathing level of selected seats under the 200 L/s supply airflow rate, 5% RH, middle injection condition. (d) Concentration profiles for m at the breathing level of selected seats under the 200 L/s supply airflow rate, 5% RH, middle injection condition. (Continued)