Exposure generated by cough released droplets in the indoor environment a comparison among four ventilation systems

Chapter 1: Introduction 1.1 Background and Motivation People in the developed countries spend more than 80 %of their time indoors (Robinson and Nelson, 1995). Many of the indoor pollutants are suspended particles in air. The human respiratory tract handles 10 l/min of air and 3000 cycles of inhalation – exhalation per day. Suspended droplets can penetrate into human respiratory system carried by inhaled air potentially causing acute or chronic health effects. Viruses have been identified as the most common cause of infectious diseases acquired within indoor environments, in particular those causing respiratory and gastrointestinal infection. Particles present in the indoor environments can originate from outdoors penetrating through building envelope drawn in by ventilation systems or can be emitted from the indoor source. Indoor source can be cooking, building materials, consumer products or occupants. Respiratory secretions from an infected person can be aerosolized through expiratory activities (breathing, talking, coughing, sneezing and vomiting) and dispersed through indoor environment. Each of these expiratory activities produces different size distribution of droplets, amount of infectious agents and initial velocities. Coughing and sneezing produces much higher number of droplets than breathing and talking although the latter two are much more frequent. These expiratory pathogen laden respiratory droplets are believed to be responsible for the epidemic spread of several respiratory tract infections. Epidemiologic studies implicate the droplet nucleus mechanism in the transmission of tuberculosis, measles, influenza, smallpox, chickenpox and SARS (Li et al., 2007). Vomiting can spread 107 virus particles per ml of vomit (Baker et al., 2001). Spread of viral infections through atomized vomit is a significant route of infection in diseases which causes frequent vomiting such as Norwalk Like Virus (NLV), also vomiting by SARS infected person on the corridor of Metropol Hotel in Hong Kong is believed to have caused a series of infections. Infected individuals can release 1012 virus particles per gram of feces (Baker et al., 2001) or up to 105 CFU/ml of bacteria. Motion of infectious droplets in a ventilated room depends on ventilation air pattern, droplet size, density, number, pollution source location, etc. Among these parameters the ventilation air pattern is the most important parameter influencing airborne infectious disease transmission in the indoor environment (Morawska 2006). Droplets in the air are subjected to Brownian forces, gravity, turbulent diffusion, inertial forces, RH, thermal gradients, electrical forces, electromagnetic radiation. Depending on the droplet size above mentioned forces have different magnitude of influences on droplets dispersion. Several technologies have been used to reduce the amount of infectious droplets in the indoor air. Ultraviolet radiation (UV) in the wavelength of 254nm is germicidal and has been used for air disinfection within indoor environment (Riley, 1972). Efficiency of UV lighting depends on: intensity of UV radiation, the species of organisms, and RH. Wavelength of UV radiation is irritating to skin and eyes and it cannot be permitted to impinge on people in doses above the limit recommended by National Institute of Occupational Safety and Health (NIOSH, 1972). Due to this restriction UV lighting is placed between occupants head and ceiling. This influences the efficiency of UV lights and technology have not been applied outside medical facilities where risk of outbreak of airborne infectious diseases is high (Bloch et al., 1985; Gustafson et al., 1982, Catanzaro, 1982). Room air filtration is another technology applied for air purification and disinfection. The effectiveness of in-room filtration depends on: single-pass filter efficiency, air flow rate through the filter and on other features of the indoor environment particularly the relative positions of the source and receptor and the indoor air flow patterns generated by air conditioning and ventilation system (Miller-Leiden et. al., 1996). Although this process have shown high efficiency to droplets with diameter less than 3μm when filter is placed close to the occupant, while for droplets with diameter of less than 1μm the efficiency is low when HEPA filter is not used. Using HEPA filters increase energy consumption necessary for fan operation to overcome high pressure drop of filter. This technology also has not found any application outside medical facilities. Two main reasons preventing usage of UV lighting and room air filtration in office building are the low efficiency and high amount of energy they require for operation. In the indoor environment where risk of infectious disease spread is not high usage of these technologies is not economically justified. Ventilation air patterns can be used to prevent airborne infectious disease spread. Several ventilations systems are currently used in office environments. These systems generate very different air patterns in the indoor environment. Mixing Ventilation (MV) is the most commonly used ventilation system, which supplies the air with high momentum to induce mixing in the room. Displacement Ventilation (DV) is ventilation system which supplies air with low velocity from the supply diffuser mounted on the floor. Air supplied in this way moves on the floor until it reaches heat sources which entrain this air into their boundary layers and due to thermal buoyancy displaces upwards. Under-floor Ventilation system (UF) supplies the air from diffusers mounted on the floor. Air is supplied with high momentum to ensure sufficient upward momentum while air mixes inside indoor environment. Personalized Ventilation (PV) is designed to deliver conditioned (cool and clean) outdoor air to the breathing zone of the occupant. The amount of inhaled personalized air has been shown to depend on the design of the Air Terminal Device (ATD), its positioning in regard to the occupant, the supply flow rate of the personalized airflow, as well as the difference between the room air and the Personalized Ventilation airflow temperature, size of target area, etc. (Faulkner et al., 1999; Melikov et al., 2002). The optimal performance for the most of the ATD has not exceeded 50–60% of clean air in each inhalation although it was pointed out by Bolashikov et al., (2003) that PV ATD can be designed to achieve100 % breathing zone air delivery effectiveness. When Personalized Ventilation is compared to total volume mechanical ventilation potential for energy savings has been demonstrated (Melikov, 2004; Sekhar et al., 2005). This technology is suitable for commercial office buildings and has potential to reduce risk of airborne infectious disease transmission in the indoor environment while reducing overall energy consumption of air conditioning and mechanical ventilation system Multidisciplinary literature review by Li et al. (2007) concluded that: ―there is no evidence/data to support specification and quantification of the minimum ventilation requirements in schools, offices, and other non-hospital environments in relation to the spread of airborne infectious diseases. The knowledge gap is obvious.‖ Quantification of ventilation rates will influence ability of a particular ventilation system to control (reduce) airborne infectious disease transmission by reducing exposure of the healthy occupant(s) to the infectious agents released by expiratory activities of the infected occupant(s). Ability of the ventilation system to control (reduce) exposure to airborne infectious agents is protective ability of a ventilation system. Since several ventilation systems are currently used protective ability of each one should be evaluated and compared to other systems. In the present literature there is no methodology proposed for evaluation and comparison of the protective ability of different ventilation systems. Several experimental studies (Qian et al., 2006; Qian et al. 2008; Nielsen et al 2009; Nielsen et al., 2010) or Computational Fluid Dynamic studies (CFD) (Li et al., 2005; Xie et al., 2009, Zhu et al., 2007; etc.) compared protective ability of two or more systems for a specific scenario, but no comprehensive comparison of protective ability of commonly used ventilation systems has been performed so far. This identifies the necessity for development of the methodology for evaluation of protective ability of ventilation system. This methodology should be then applied to compare different ventilation systems for the same supply flow rate as well as variation of protective ability of any of the system with variation of the supply flow rate. Occupant density needs to be included into the analysis because disease propagation (number of new cases) will depend on the number of occupants in the indoor environment under consideration. It is important to establish relationship between number of occupants in the indoor environment, ACH generated by the particular ventilation system and prevention of the airborne transmitted disease propagation. Previous studies examined ventilation efficiency of different system and provided knowledge regarding mechanisms (flow field generated) of air delivery to the breathing zone deployed by different systems. These studies were performed under the steady state using tracer gas (SF6) to quantify different ventilation indices. Cough release is episodic event which generates unsteady state in the environment. When cough is released saliva droplets (with or without infectious agents) move through the environment due to high initial momentum (cough velocity can be up to 22 m/s, while average cough velocity is 10 m/s) before sufficient momentum decay occurs and ventilation streamlines reestablish and start to carry them. There were no previous comprehensive experimental studies about protection mechanisms (flow field generated) engaged by different ventilation systems when cough release occurs in the indoor environment. Since cough droplets have high initial momentum, distance between infected occupant (infector) and exposed healthy occupant are important parameters that impact on the flow field generated by cough that will cause exposure. No previous studies have been conducted to establish influence of the infectorsusceptible distance on the exposure generated by infectors cough release and flow fields generated to provide protection of the susceptible occupant using the multiphase flow approach to simulate cough droplets. Study performed by Melikov et al., (2009) investigated influence of distance between infected coughing person and exposed person using tracer gas to simulate cough release. In order to conduct a study of the cough released droplets exposure changes due to changes of the ventilation system experiments need to be conducted in the indoor environment capable of changing ventilation modes. Cough release need to be simulated as a multiphase flow because decay of the cough velocity is much more intensive when only gas phase (commonly simulated using SF6) is used. Liquid droplets are able to preserve higher momentum and reach larger distances from the source for the same boundary conditions compared to gas phase. Simulated saliva or real human saliva needs to be used to properly simulate nonvolatile saliva characteristics. Cough release droplets can evaporate only to droplet nuclei size which than cause exposure, but when, for example, water is used to simulate cough droplets evaporation is complete and there is no nonvolatile residue. When gas (SF6) is used to simulate cough evaporation is neglected. To assess the risk of airborne infectious disease transmission, exposure of the occupants to cough released droplets need to be measured. This might be achieved by using a Breathing Thermal Manikin (BTM) to simulate convective boundary layer generated around heated body and inhalation – exhalation flow in the breathing zone. Size distribution of cough droplet and droplet nuclei need to be measured in the breathing zone of the BTM to estimate the risk of infection because different droplet sizes have different deposition characteristics in different parts of the respiratory track. Some of the previous experimental studies (Chao and Wan 2006; Wan and Chao 2007; Qian et al.; 2006; Cermak and Melikov 2007; Qian et al., 2008; Chao et al. 2008; Nielsen 2009) were conducted with some of the above mentioned equipment, but no study so far used all these equipment to study changes of the exposure to cough released droplets with the application of different ventilation systems. Epidemiological investigation of airborne infectious disease transmissions in the indoor environment (e.g. Riley et al., 1978; Catanzaro 1982; Nardell et al., 1991; Nicas 2000) is usually faced with several uncertainties. Occupants (infector and all susceptible occupants) move in the indoor environment and change distances, heights (sitting or standing) and position (facing each other; back to back, or any other possibility in between) in respect to each other. Changes of these parameters distance, height and position will cause changes of the susceptible occupants‘ exposure, but these changes are generally unknown. When a particular air delivery system is used (e.g. MV) type of supply diffusers (perforated, jet and multi-nozzle, conical diffusers, swirl and rectangular 4-way), size of return grilles, positions and number of supply diffusers and return grilles relative to each other will influence air patterns and turbulence levels generated in the indoor environment. When cough is released these air patterns interact with multiphase cough flow and influence dispersion of potentially infectious cough droplets. Different interactions among multiphase cough flow and generated air patterns will cause different levels of exposure of susceptible occupants. Although generated air patterns under steady state environment can be known, due to changes of the distance, height and position of infector and susceptible occupants‘ exposures of the occupants are generally unknown. Another important parameter influencing exposure is distribution of cough releases (frequency) throughout exposure time, which is generally also unknown. In order to simplify investigation and overcome these uncertainties assumption of perfectly mixed environment and steady release of potentially infectious expiratory droplets are commonly used. These assumptions can lead to large risk underestimation (Nicas, 1996). Wells-Riley approach (Riley et al., 1978) or modifications of the original equation incorporating other effects (Nazaroff et al., 1998; Rudnick and Milton 2003; Fisk et al, 2005; Noakes 2006; Noakes and Sleigh 2009) was used in several studies to calculate risk of airborne infectious disease transmission. Although some of the studies (Noakes and Sleigh 2009) divided indoor environment into zones, each of the zones was treated as perfectly mixed while source was treated as steady constant release which is simplification of the episodic cough release occuing in the real environment. Wells-Riley equation needs to be augmented to cater for heterogeneity and unsteadiness in the indoor environment in order to be applied for evaluation of risk of airborne infection for different air delivery systems and various supplied flow rates. 1.2 Research Objectives The aim of this research is to evaluate ability of different ventilation systems to provide protection to the susceptible occupant form the airborne infectious disease transmission due to cough released infectious agents. The objectives are as follows: 1. Develop a methodology to evaluate the efficiency of reduction in concentration of cough released droplets in the breathing zone of the occupant and apply this methodology to evaluate protective ability of four ventilation systems at various air supply flow rates. 2. Evaluate the validity of perfectly mixed assumption in the breathing zone commonly used in estimating the probability of getting infected for different ventilation systems at different air flow rates. 3. Augment the Wells-Riley equation for unsteady heterogenic environment and demonstrating its applicability to different ventilation systems and infectious loads. 4. Examine how the protective performance of different ventilation systems vary with the height of cough release (sitting and standing position of the infector) and distance between infected and susceptible occupant. 5. Examine flow field characteristics of the interactive two-phase flows between cough released droplets and room air flow generated with different ventilation systems. 1.3 Scope of Work This study is in the field of ventilation but results from this study have application on medical problem of control of airborne infection disease transmission. The scope of work and the structure of discussion in each chapter are described briefly as follows:  Literature review. This study is not independent of, but based on, previous research on motion of expiratory droplets in indoor environment. Chapter provides useful information on experimental design used to study the dispersion of expiratory droplets and methods adopted to evaluate risk of airborne infectious disease transmission.  Research methodology. The work comprised a series of three related studies: (i) development and application of methodology for evaluation of overall protective ability of different ventilation systems and reduction of assumption necessary to evaluate probability of getting infected using Wells-Riley approach; (ii) evaluation of distance between infector and susceptible on the protection from cough released infectious droplets achieved with different ventilation systems; (iii) investigation of flow field characteristics of the resulting flow field between cough released droplets and room air flow generated influencing reduction droplet concentration in the breathing zone. The research methodology is described for the series of experiments, which includes experimental design (facility and instrument) and data analysis (data and statistical analysis). The discussion of research methodology is presented in Chapter 3.  Chapter describes the overall influence of ventilation system on control of airborne infectious disease transmission. A evaluation methodology is proposed for the calculation of overall averaged probability of getting infected for a susceptible occupant. The method is adopted to two different risk assessment models for the indoor environment supplied with a particular ventilation system. These results were than compared to calculations conducted with the perfectly-mixed assumption, and Basic Reproductive number variations are shown for different levels of occupancy.  Chapter describes the influence of distance between infector and susceptible on the protection from cough released infectious droplets achieved with ventilation. The influence of protective performance of different ventilation systems on height (sitting and standing position of the infector) and distance between infected and susceptible occupant was studied.  Chapter (and Appendix and 2) documents the potential exposure and flow field characteristics generated by direct cough at different infector - susceptible distances in the indoor environment supplied with various ventilation systems. The interaction between cough and the flow fields generated by different ventilation systems and their consequent impact on exposure from the direct cough at several infector – susceptible distances were studied.  Conclusion and Recommendation. The objectives are reviewed and a summary of significant findings is presented. These include: (i) the contributions of the new methodology for evaluation of ability of ventilation system to control airborne infection disease spread; (ii) the validity of the proposed augmentation of Wells-Riley equation; (iii) evaluation of protective performance of different ventilation systems (overall and infector-susceptible distance dependant); and (iv) flow field characteristics responsible for protection of susceptible occupant. Lastly, some suggestions for further research and the development of ventilation system for airborne infection transmission control are given in Chapter 7. 10 Droplets are than entrained by CBL and brought into the breathing zone. In less than 0.1 s UF streamlines are re established carrying droplets upwards with the velocity up to 0.4 m/s. Results for 12 ACH for UF system indicate that flow field generated is very similar to the ACH case (APPENDIX VOL V- field UF 12 ACH sitting i-s distance 2m). When cough is released at i-s distance of m on line from the sitting height, droplet momentum is reduced after impingement upon the back of manikin‘s head (APPENDIX VOL V- field UF ACH sitting i-s distance 3m). This momentum reduction causes downward motion of droplets due to gravity. Downward motion disturbs UF streamlines but they re establish quickly and start to move droplets upwards. Due to action of turbulence, droplets mix while being carried with UF streamlines and reach breathing zone in a very low concentration. Some of the droplets are entrained by CBL due to action of two vortices generated in the horizontal plane on the left and right side of the face, but these vortices have a very short life time. When UF is supplying 12 ACH while cough is released at i-s distance of m on line from the sitting height, UF streamlines coupled with CBL prevent droplets from reaching breathing zone by displacing droplets upwards after impingement on the back of manikin‘s head (APPENDIX VOL V- field UF 12 ACH sitting i-s distance 3m). 283 A.3.7 Potential exposures generated with DPV system Concentration [particles/L] 400000 350000 300000 250000 200000 150000 100000 50000 0 10 15 20 25 30 Time after cough release [s] 1m position ach 3m position ach 1m position 12 ach 3m position 12 ach 2m position ach 4m position ach 2m position 12 ach 4m position 12 ach Concentration [particles/L] Figure A1.3.7. Cumulative potential exposure of cough released droplets dp≤1μm for DPV at and 12 ACH at 1, 2, and m. 30000 20000 10000 0 10 15 20 25 30 Time after cough release [s] 1m position ach 3m position ach 1m position 12 ach 3m position 12 ach 2m position ach 4m position ach 2m position 12 ach 4m position 12 ach Figure A1.3.8. Cumulative potential exposure of cough released droplets dp≥1μm for DPV at and 12 ACH at 1, 2, and m. When DPV is used potential exposure generated with ACH compared to potential exposure generated for 12 ACH. For the i-s distances of m and m DPV jet prevent droplets from coming into breathing zone. Potential exposure generated at m is reduced by 15 % for d p≤1μm 284 and by % for dp≥1μm when ACH is used compared to m case. When 12 ACH is used, potential exposure is reduced by 27 % for dp≤1μm and by 18 % for dp≥1μm compared o the potential exposure generated for m. A.3.8 Line Cough Velocity Profiles for DPV system and sitting height of release After cough impingement on the back of manikin‘s head, DPV flow carry cough droplets to the upper zone of the room when DPV is supplying ACH (APPENDIX VOL V- field DPV ACH sitting i-s distance 1m). MV streamlines start to carry droplets and exhaust them. No exposure occurs due to influence of the DPV flow. When DPV is supplying 12 ACH while cough is released at i-s distance of m on line from the sitting height, DPV flow prevents generation of vortex motion in front of the manikin‘s face after cough flow impingement on the back of manikin‘s head (APPENDIX VOL V- field DPV 12 ACH sitting i-s distance 1m). Exposure occurs due to mixing of the cough droplets in the surrounding (droplets carried by background MV streamlines or moved downward by gravity after cough momentum sufficiently decayed) with DPV flow, but concentration reaching breathing zone is highly diluted. Due to dispersion after cough flow impingement on the back of manikin‘s head some of the droplets are entrained by DPV streams generated with two ATD (APPENDIX VOL V- field DPV ACH sitting i-s distance 2m). These droplets are than carried with DPV streams into the breathing zone. Results indicate similar droplet motion to ACH case (APPENDIX VOL V- field DPV 12 ACH sitting i-s distance 2m). 285 When DPV is used with or 12 ACH cough droplets cause low exposure due to mixing with DPV flow (APPENDIX VOL V- field DPV and 12 ACH sitting i-s distance 3m). A.3.9 Relationship between background concentration produced by MV and impact of DPV jet When cough was released along line while supply flow rate was ACH at the i-s distance of m, DPV generates 69 % lower potential exposure for dp≤1μm and 14 % for dp≥1μm than MV (Table 1.12). When 12 ACH is supplied, for the i-s distance of m lower potential exposure is generated with DPV for dp≤1μm by 225 % and for dp≥1μm by 56 %. When i-s distance is increased to m for ACH, potential exposure is lower by 288 % for d p≤1μm and for 88 % for dp≥1μm. This result suggests that PV jets prevent droplets from entering breathing zone due to vortex motion on the left and right side of the manikin‘s head while entrainment of PV jets is insufficient to cause and substantial potential exposure increase. Table A1.12. Comparison of potential exposures generated with MV and DPV. ACH 12 ACH tE )C(PV )] [C(MV i0 i i tE dp≤1μm dp≥1μm dp≤1μm dp≥1μm 69 288 14 88 225 / 56 / ) Ci(MV i0 i-s distance m i-s distance m A.3.10 Influence of supply flow rate increase on potential exposure generated with different air delivery systems When supply flow rate is increased when air is delivered with MV for i-s distance of m potential exposure is reduced by % for dp≤1μm and 13 % for dp≥1μm. For the i-s distance of 286 m potential exposure is reduced by 53 % for all droplet sizes (Table 1.13). These results imply that ventilation increase has more impact on the potential exposure with the increase of i-s distance. When air is supplied with DV for i-s distance of m potential exposure is reduced by 10 % for dp≤1μm and by 17 % for dp≥1μm (Table 1.13). When i-s distance is increased to m potential exposure is reduced by 22 % for dp≤1μm and by 25 % for dp≥1μm. These results imply that ventilation increase had more pronounced influence on reduction of potential exposure with the increase of i-s distance. For the i-s distance of m when UF supplied the air increase of the supply flow rate increased potential exposure by 126 % for dp≤1μm and by 134 % for dp≥1μm. When i-s distance was increased to m increase of the supply flow rate caused reduction of potential exposure by 13 % for dp≤1μm and by % for dp≥1μm. When i-s distance was further increased to m increase of the supply flow rate caused reduction of potential exposure by 37 % for d p≤1μm and by 38 % for dp≥1μm. These results imply that increase of the supply flow rate can increase or reduce potential exposure depending on the i-s distance. When DPV supplies the air for i-s distance of m potential exposure is reduced by 22 % for dp≤1μm and 31 % for dp≥1μm. For i-s distance of m increase of the supply flow rate caused reduction of potential exposure by 69 % for dp≤1μm and by 72 % for dp≥1μm. When i-s distance was further increased to m increase of the supply flow rate caused reduction of potential exposure by 33 % for dp≤1μm and by 31 % for dp≥1μm. These results imply that maximum potential exposure reduction with the supply flow rate increase is achieved for i-s distance of m. Table A1.13. Influence of supply flow rate increase on potential exposure for sitting height 287 tE [C(6ach )C(12 ach )  i0 i MV i DV tE C(6ach )  i0 i UF DPV 1m 13 / / -126 -134 22 31 dp≤1μm dp≥1μm dp≤1μm dp≥1μm dp≤1μm dp≥1μm dp≤1μm dp≥1μm 2m 53 53 10 17 13 69 72 3m / / 22 25 37 38 33 31 A.4. Standing position of the cough release for Line A.4.1 Potential exposures generated with MV system Concentration [particles/L] 60000 50000 40000 30000 20000 10000 0 10 15 20 25 30 35 40 Time after cough release [s] 1m position ach 1m position 12 ach 2m position ach 2m position 12 ach 3m position ach 3m position 12 ach 4m position ach 4m position 12 ach Figure A1.4.1. Cumulative potential exposure of cough released droplets dp≤1μm for MV at and 12 ACH at 1, 2, and m. 288 Concentration [particles/L] 10000 8000 6000 4000 2000 0 10 15 20 25 30 Time after cough release [s] 1m position ach 1m position 12 ach 2m position ach 2m position 12 ach 3m position ach 3m position 12 ach 4m position ach 4m position 12 ach Figure A1.4.2. Cumulative potential exposure of cough released droplets dp≥1μm for MV at and 12 ACH at 1, 2, and m. When cough is released from the standing height along line while MV system is used for air distribution potential exposure generated at ACH is higher than potential exposure generated with 12 ACH (Figure 6.8.1 and 6.8.2). At i-s distance of m and m cough droplets not cause potential exposure. For the i-s distance of m potential exposure generated for ACH for dp≤1μm is reduced by 43 % and for dp≥1μm is reduced by 46 % compared to m case. When 12 ACH is used, potential exposure generated at i-s distance of m reduced by 28 % for dp≤1μm, but increased by 15 % for dp≥1μm. A.4.2 Line Cough Velocity Profiles for MV system for standing height of release When MV is supplying ACH while cough is released at i-s distance of m on line from the standing height, droplets reach manikin with downward motion (APPENDIX VOL V- field MV ACH standing i-s distance 2m). Downwards motion of droplets causes initial exposure by blowing-off CBL and at the same time generating vortex in front of manikins face. CBL re establishes 0.3 s after initial exposure while 0.42 s is necessary for MV streamlines to re establish 289 and start to move droplets upwards. Upward displacement of cough droplets with joint action of CBL and MV streamlines rapidly reduces concentration of droplets in the breathing zone. When MV is supplying 12 ACH while cough is released at i-s distance of m on line from the standing height, after impingement on the manikin head droplets start to move downwards along manikin‘s face blowing-off CBL in the breathing zone (APPENDIX VOL V- field MV 12 ACH standing i-s distance 2m). Vortex is generated below manikin‘s chin while the rest of the flow field is dominated by downward droplet motion. Flow field above the head restores its original direction and causes vortex to move in the direction away from the manikin‘s face. This causes very chaotic droplet motion in the breathing zone where gravity and residual vortex motion interchange dominant influence onto droplet motion. s after initial exposure CBL re establishes carry droplets upwards diluting the breathing zone. When MV is supplying ACH while cough is released at i-s distance of m on line from the standing height, cough droplets pass above manikin‘s head with velocity of 0.8 m/s (APPENDIX VOL V- field MV ACH standing i-s distance 3m). Droplets next to the manikin‘s head start to move downwards and blow off CBL from the breathing zone. CBL in re establishes in less than 0.1 s. Cough droplets still maintain dominantly downward motion mixing with CBL due to motion in the opposite direction for 0.2 s. Vortex is then generated in the zone in front of manikin‘s face. Vortex moves away from the manikin‘s face in the direction of cough flow causing reduction of droplet concentration in the breathing zone. When MV is supplying 12 ACH while cough is released at i-s distance of m on line from the standing height, cough droplets pass above manikin‘s head with velocity in the range of 0.4 to 0.7 m/s (APPENDIX VOL V- field MV 12 ACH standing i-s distance 3m). This velocity is lower that observed for ACH case. CBL in the breathing zone is present during entire exposure 290 period. Vortex is generated in front ot he manikin‘s face, but with the very short life time. MV streamlines in the zone in front of the manikin‘ face re establish in less than 0.1 s after initial exposure caused by entrainment of droplets by CBL. MV streamlines and CBL remove droplets upwards with the velocity up to 0.5 s causing significant reduction of concentration in the breathing zone. When MV is supplying or 12 ACH while cough are released at i-s distance of m on line from the standing height, cough droplet momentum decays significantly before reaching manikin (APPENDIX VOL V- field MV and 12 ACH standing i-s distance 4m). Droplets are then carried by MV streamlines upwards and exhausted them from the indoor environment. A.4.3 Potential exposures generated with DV system Concentration [particles/L] 80000 70000 60000 50000 40000 30000 20000 10000 0 10 15 20 25 30 35 40 Time after cough release [s] 1m position ach 1m position 12 ach 2m position ach 2m position 12 ach 3m position ach 3m position 12 ach 4m position ach 4m position 12 ach Figure A1.4.3. Cumulative potential exposure of cough released droplets dp≤1μm for DV at and 12 ACH at 1, 2, and m. 291 Concentration [particles/L] 12000 10000 8000 6000 4000 2000 0 10 15 20 25 30 35 40 Time after cough release [s] 1m position ach 1m position 12 ach 2m position ach 2m position 12 ach 3m position ach 3m position 12 ach 4m position ach 4m position 12 ach Figure A1.4.4. Cumulative potential exposure of cough released droplets dp≥1μm for DV at and 12 ACH at 1, 2, and m. For cough release from the standing height along line potential exposure generated with ACH is higher than for 12 ACH when DV is used (Figure 1.4.3 and 1.4.4). When 12 ACH is supplied potential exposure is generated only for i-s distance of m, while for ACH exposure is generated for i-s distances of 2m, 3m and 4m. Increase of the i-s distance had minor (reduction less than %) impact on the exposure reduction. A.4.4 Line Cough Velocity Profiles for DV system for standing height of release When DV is supplying 12 ACH while cough is released at i-s distance of m on line from the standing height, vortex is generated after droplets pass above manikin‘s head (APPENDIX VOL V- field DV 12 ACH standing i-s distance 2m). Vortex motion causes droplets to enter breathing zone while CBL is blown-off. 0.3 s after initial exposure CBL re establishes and start to displace droplets upwards. When DV is supplying ACH while cough is released at i-s distance of m on line from the standing height, cough droplets reach manikin‘s head with velocity in the range of 0.3 to 0.5 m/s 292 (APPENDIX VOL V- field DV ACH standing i-s distance 3m). Droplets next to manikin‘s head move downwards and blow off CBL causing initial exposure. CBL is re establishes in less than 0.1 s and start to dilute breathing zone by displacing droplets upwards. When MV is supplying 12 ACH while cough is released at i-s distance of m on line from the standing height, cough droplets reach manikin‘s head with velocity lesser than 0.35 m/s (APPENDIX VOL V- field DV 12 ACH standing i-s distance 3m) CBL interacts with cough flow and start to displace droplets upwards with velocity 0.2 m/s. When DV is supplying or 12 ACH while cough are released at i-s distance of m on line from the standing height, cough droplet momentum decays significantly before reaching manikin (APPENDIX VOL V- field DV and 12 ACH standing i-s distance 4m). Droplets are then displaced upwards with CBL. A.4.5 Potential exposures generated with UF system Concentration [particles/L] 150000 120000 90000 60000 30000 0 10 15 20 25 30 35 40 Time after cough release [s] 1m position ach 1m position 12 ach 2m position ach 2m position 12 ach 3m position ach 3m position 12 ach 4m position ach 4m position 12 ach Figure A1.4.5. Cumulative potential exposure of cough released droplets dp≤1μm for UF at and 12 ACH for 1, 2, and m. 293 Concentration [particles/L] 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 10 15 20 25 30 35 40 Time after cough release [s] 1m position ach 3m position ach 1m position 12 ach 3m position 12 ach 2m position ach 4m position ach 2m position 12 ach 4m position 12 ach Figure A1.4.6. Cumulative potential exposure of cough released droplets dp≥1μm for UF at and 12 ACH for 1, 2, and m. When cough is released form the standing height along line while UF system is used for air distribution potential exposure generated with ACH is higher than potential exposure generated with 12 ACH (Figure 1.4.5 and 1.4.6). Potential exposure is generated for i-s distances of m and m for both supply flow rates examined. For i-s distances of and m potential exposure for all droplet sizes and supply flow rates examined is similar. A.4.6 Line Cough Velocity Profiles for UF system for standing height of release When UF is supplying ACH while cough are released at i-s distance of m on line from the standing height, cough droplets reach manikin‘s head with velocity in the range of 0.7 to 0.9 m/s (APPENDIX VOL V- field UF ACH standing i-s distance 2m). After reaching manikin‘s head droplets start to move downwards and generate vortex in front of manikin‘s face. While moving downwards droplets also blow off CBL in the breathing zone for 0.1 s causing initial exposure. Vortex moves away from the manikins face in the direction of the cough flow while UF streamlines are re establish and remove droplets upwards diluting the breathing zone. 294 Velocity and concentration profile (APPENDIX VOL for C and V field UF 12 m i-s front). When UF is supplying 12 ACH while cough are released at i-s distance of m on line from the standing height, cough droplets start to move downwards before reaching manikin‘s head due to more intensive momentum decay compared to ACH case (APPENDIX VOL V- field UF 12 ACH standing i-s distance 2m) While moving downwards droplets completely blow off CBL and cause exposure of the manikin. 0.3 s after initial exposure CBL re establishes and start to dilute breathing zone by displacing droplets upwards. When droplet motion is compared for UF supply of and 12 ACH, they show very different mechanism causing initial exposure and dilution of the breathing zone. When UF is supplying 12 ACH while cough are released at i-s distance of m on line from the standing height, cough droplets start to move downwards before reaching manikin‘s head (APPENDIX VOL V- field UF ACH standing i-s distance 3m). Droplets blow off CBL while moving downwards and cause initial exposure. CBL re establishes in 0.15 s. and due to counter flow direction with downward moving droplets vortex is generated in front of manikin‘s face. 0.5 s after initial exposure CBL upward flow dominate zone in front of manikin‘s face. 0.6 s after initial exposure UF streamline re establish and start to carry droplets upwards from the region of the flow not affected by CBL. This upward droplet motion with the velocity up to 0.5 m/s produced by coupled influence of CBL and UF streamlines cause rapid removal of droplets from the breathing zone. Cough droplets start to move downwards before reaching manikin‘s head when UF is supplying 12 ACH while cough are released at i-s distance of m on line from the standing height (APPENDIX VOL V- field UF 12 ACH standing i-s distance 3m). While moving downwards cough droplets blow off CBL in the breathing zone for less than 0.1 s causing initial exposure. 295 0.3 s after initial exposure UF streamlines re establish and start to carry cough droplet upwards causing reduction of concentration in the breathing zone. Flow field in established in front of manikin‘s face carry droplets upwards due to joint action of UF streamline and CBL with the velocity in the range of 0.2 to 0.6 m/s. When UF is supplying or 12 ACH while cough are released at i-s distance of m on line from the standing height, cough droplet momentum decays significantly before reaching manikin (APPENDIX VOL V- field UF and 12 ACH standing i-s distance 4m). Droplets are then carried by UF streamlines upwards and exhausted. A.4.7 Line Cough Velocity Profiles for DPV system for standing height of release When DPV is supplying or 12 ACH while cough are released at i-s distance of 2, or m on line from the standing height, cough droplets are blown-off by DPV flow before reaching breathing zone (APPENDIX VOL V- field DPV and 12 ACH standing i-s distances 2m, 3m and 4m). A.4.8 Influence of supply flow rate increase on potential exposure generated with different air delivery systems When supply flow rate is increased from to 12 ACH, for MV at i-s distance of m potential exposure is reduced by 58 % for dp≤1μm and by 65 % for dp≥1μm. When i-s distance is increased to m potential exposure is reduced by 20 % for dp≤1μm and by 24 % for dp≥1μm. These results suggest that for standing height of cough release increase of ventilation has the highest impact at i-s distance of m. 296 For the DV increase of supply flow rate from to 12 ACH reduced potential exposure by 77 % for all droplet sizes at i-s distance of m. Among all ventilation systems this is the highest potential exposure reduction caused by supply flow rate increase. When supply flow rate is increased from to 12 ACH, for UF at i-s distance of m potential exposure is reduced by 22 % for dp≤1μm and by % for dp≥1μm. For the i-s distance of m potential exposure is reduced by 47 % for dp≤1μm and by 25 % for dp≥1μm. These results suggest that increase of supply flow rate causes the highest reduction of potential exposure oa i-s distance of m for UF system. When results for all TVS are compared they show that the highest potential exposure reduction due to increase of supply flow rate will depend on the i-s distance for different systems. Table A1.14. Influence of supply flow rate increase on potential exposure for standing height tE [C(6ach )C(12 ach )]  i0 i i tE C(6ach )  i0 i MV DV UF dp≤1μm dp≥1μm dp≤1μm dp≥1μm dp≤1μm dp≥1μm 2m 58 65 77 77 22 3m 20 24 / / 47 25 Tables A1.15, A1.16 and A1.17 show the statistics of exposure levels calculated from the 10 measurements for every point, distance and height of cough release. Statistical significance test (t-test) was performed to compare the exposure levels among cases for different ventilation systems. All the t-values are greater than the critical t-value at 95 per cent confidence interval, indicating that the reduction in exposure by the PV device is statistically significant in all the examined scenarios. 297 Table A1.15. The mean, s.d. and range of exposure levels to the expiratory aerosols calculated for Mixing and Displacement ventilation. Mixing ventilation Displacement ventilation distance mean s.d. range mean s.d. range [m] exposure [ml] [ml] exposure [ml] [ml] [ml] [ml] 2.37 x 10-4 4.12 x 10-5 1.76 x 10-4 2.20 x 10-4 4.34 x 10-5 1.83 x 10-4 -4 3.23 x 10 3.11 x 10-4 1.98 x 10-4 3.53 x 10-5 2.38 x 10-4 2.75 x 10-4 3.66 x 10-5 2.06 x 10-4 -4 3.98 x 10 3.32 x 10-4 -4 -5 -4 -4 -5 1.86 x 10 2.18x 10 1.12 x 10 1.62 x 10 2.08x 10 1.02 x 10-4 -4 2.16 x 10 2.03 x 10-4 -5 -5 -5 -5 -5 7.21 x 10 1.31 x 10 5.78 x 10 7.14 x 10 1.20 x 10 5.68 x 10-5 8.73 x 10-5 7.91 x 10-5 Table A1.16. The mean, s.d. and range of exposure levels to the expiratory aerosols calculated for Under-floor and Personalized ventilation. Under-floor ventilation Personalized ventilation distance mean s.d. range mean s.d. range [m] exposure [ml] [ml] exposure [ml] [ml] [ml] [ml] 2.77 x 10-4 4.52 x 10-5 2.13 x 10-4 1.60 x 10-4 3.28 x 10-5 1.35 x 10-4 -4 3.18 x 10 2.31 x 10-4 -4 -5 -4 -4 -5 3.24 x 10 3.76 x 10 2.50 x 10 1.27 x 10 2.77 x 10 8.03 x 10-5 -4 4.22 x 10 1.61 x 10-4 -4 -5 -4 -5 -5 2.15 x 10 2.18 x 10 1.34 x 10 6.67 x 10 1.56 x 10 9.37 x 10-5 2.63 x 10-4 3.62 x 10-5 -5 -5 -5 -5 -5 7.67 x 10 1.38 x 10 6.24 x 10 5.33 x 10 1.14 x 10 1.78 x 10-6 -5 8.81 x 10 2.85 x 10-5 Table A1.17. Results of statistical significance test (t-test) on the reduction of exposure level by PV. Critical t-value for 95 per cent confidence interval is 2.10. distance [m] t-value p-value 10.21 [...]... the exhaled bioaerosols in the study of influenza transmission 2.5.2 Inhalation intake fraction Although there is variation in the way these terms are defined across the many disciplines involved in environmental health, most accept that exposure represents the contact between an agent and a target‖ In contrast, ―dose‖ is the amount of pollutant that is absorbed by a target‖ In 33 addition, the term... in time and space People are exposed to pollutants when they encounter these concentration fields Intake represents the inhalation of contaminants owing to their presence in the breathed air For the inhalation pathway, the intake fraction is defined as the attributable mass of a pollutant inhaled per unit mass released 2.5.3 Imperfectly Mixed Indoor Environments Incomplete mixing can be important in. .. transverse to, assisting or opposing the transient 20 flow of exhalation, the buoyancy driven boundary flow and the ventilation flow are of major importance for minimizing mixing in spaces With careful consideration of these factors affecting airflow patterns, pathogen laden droplets can be removed faster from the indoor air either exhausting them faster or increasing the deposition rate Available knowledge... situations The potentially infectious droplets exhaled from the patients with pulmonary tract infection can cause cross infection especially in hospitals Performance of wall mounted and ceiling mounted downward mixing ventilation and displacement ventilation system in hospital ward was examined by Qian et al (2006) using a personal exposure index (defined as the ratio between pollutant concentration in the. .. mixing, the supply air was incapable of pushing down the source patient‘s exhaled contaminants, and produced a downward unidirectional flow It was pointed out that results in the study by Qian et al (2008) are only qualitative and they only represent the trends since droplets are simulated by tracer gas, physics of large droplets was fully neglected as well as deposition, evaporation and coagulation... humidity of the ambient air Expelled large droplets were carried more than 6 m away by exhaled air at a velocity of 50 m/s (sneezing), more than 2 m away at a velocity of 10 m/s (coughing), and less than 1 m away at a velocity of 1 m/s (breathing) In the study of respiratory droplets movement in indoor environment using CFD Lai and Cheng (2007), simulated two standing persons at 1m distance in mixing and displacement... parameters that can be determined in tracer gas ventilation studies include the room mean age of air, the local mean age of contaminant, the room mean age of contaminant and the mean residence time of contaminant (Sandberg and Sjoberg, 1983) Various measures of ventilation efficiency based on these parameters have been proposed (Skaaret, 1986), but none permits to estimate exposure intensity per se (Nicas,... dispersion in the initial dispersion stage and much slower dispersion in the later stable stage The large size group droplets or droplet nuclei had only initial dispersion stage behavior due to rapid settling Extraction by following the exhaust air stream was the major removal mechanism (over 50%) for the small size group and gravitational settling was the dominant removal mechanism (over 98%) for the large... dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance Microorganisms carried in this manner may be dispersed over long distances by air currents and may be inhaled by susceptible individuals who have not had face-to-face contact with (or been in the same room with) the infectious individual... secondary airborne infection by Nicas et al (2005) was proposed in such a way that expelled particles with da≤20μm have the ability to evaporate quickly and constitute the inhaled particles with da≤10μm Because the rate of particle removal from room air and the probability of particle deposition in the alveolar region depends on particle diameter, Nicas et al (2005) separately accounted for the particles . mixing ventilation and displacement ventilation system in hospital ward was examined by Qian et al. (2006) using a personal exposure index (defined as the ratio between pollutant concentration. parameters the ventilation air pattern is the most important parameter influencing airborne infectious disease transmission in the indoor environment (Morawska 2006). Droplets in the air are subjected. occupants in the indoor environment under consideration. It is important to establish relationship between number of occupants in the indoor environment, ACH generated by the particular ventilation