Control of Indoor Airflows for Reduction of Human Exposure to Aerosol Contaminants

Abstract

Air distribution in indoor environments is a critical factor of occupants’ exposure to airborne contaminants. There is a wide range of gaseous and biological contaminants which deteriorate the indoor air quality and thus affect negatively occupants’ health and performance. Increasing attention is being paid to analysing indoor airflow patterns and on understanding indoor pollution transmission to the breathing zone of occupants. However, studies rarely take into account the complex airflow interaction in the breathing zone, which may lead to inaccurate exposure prediction. Therefore, there is still a need for improved understanding of the air movement in the vicinity of the occupants. Tracer gas measurements are often used to study exposure to both indoor generated gases and airborne particles (aerosols). The tracer gas, however, cannot be used as a common substitute for aerosols of all sizes due to the different physical forces acting on them. Determining to what extent tracer gas can be used as substitute for aerosols when assessing occupants’ exposure to indoor aerosols is needed and can be used for appropriate ventilation systems design. A properly developed ventilation method achieves the maximum efficiency with the minimum airflow rate, avoiding excessive installation and maintenance costs and more importantly, excessive energy use can be avoided. It is well-known that the most efficient method to prevent the risk of exposure is to control the contaminants directly or close to their source. A person, particularly his/her body, may be the primary source of unpleasant and even contagious contaminants in spaces. Dilution of the contaminated room air by supply of clean air, known as ventilation by dilution, is a recognised method for improving indoor air quality. The current method for ventilating an entire room based on total volume air distribution principles is often not efficient in providing high quality environment and satisfying every occupant. Hence, local exhaust ventilation applied in the vicinity of the occupants, i.e. close to the pollution source can offer a better solution. The main objectives of the present thesis are: 1) to study the effect of typical airflows interactions around the human body (convective boundary layer, respiratory flow, and flow of local ventilation flow) on transport mechanisms of airborne contaminants and the resulting occupants’ exposure; 2) to verify the use of tracer gas as a measure of exposure to indoor aerosols; 3) to develop and study local exhaust ventilation methods for exposure reduction to body-emitted contaminants in indoor environments. The most important findings of the research performed in this thesis are summarized in the following: In ventilated rooms with low air mixing, the interaction of the exhaled flow with the convective boundary layer (CBL) around a seated person increases the exposure to own body released pollution, especially when the pollution is generated close to the breathing zone. Breathing does not affect exposure to gaseous pollutants emitted from the lower part of the body. Local airflow from personalised ventilation directed against the face with mean air speed of 0.4 m/s can reduce substantially the exposure regardless of the pollution source location. However, when the personalised airflow is combined with local source control, i.e. local exhaust of pollution, the exposure may increase depending on the airflow interaction at the breathing zone and the source location. Exposure assessment based on tracer gas concentration measurement can be incorrect if the measuring instrument has long response time and the complex airflow interaction in the breathing zone is not correctly simulated. Results showed that in the breathing zone of a seated occupant, the tracer gas emerged as a reliable predictor for the exposure to aerosols with aerodynamic diameter 0.07, 0.7, and 3.5 μm in a room with mixing air distribution. An increase of the air change rate did not affect the comparable normalized concentration distribution of the tracer gas and the larger particles, namely 0.7μm and 3.5 μm. However, the ventilation rate was important for comparing the behaviour of the ultrafine particles (0.07 μm) and the tracer gas in the breathing zone. A moderate change of the room surface area did not influence the resemblance in the dispersion of the aerosols and the tracer gas. The results also showed that tracer gas can be used to indicate the exposure of a person lying in bed to 0.7 μm aerosols. Furniture-integrated exhaust methods can be used as a pollution source control strategy in facilities where people are seated or bed-bound for considerable amounts of time. The current study examined ventilated mattress and ventilated seat cushion as local pollution exhaust methods. It was found that at reduced background ventilation rate, the use of the ventilated mattress and the ventilated seat cushion improved the air quality substantially when the pollution source was located near the exhaust openings. The pollution was removed from the room through the ventilated mattress or seat cushion’s connection with the exhaust system before it was mixed with the room air. An alternate approach was to install a filter inside the mattress in order to clean the exhausted air of body effluents and recirculated it back into the room. This provides flexibility of bed location (the bed with own ventilation can be moved to ventilated or non-ventilated rooms) and avoids installation of additional ducting. This technique can also be applied in the case of the ventilated seat cushion. The ventilated mattress and seat cushion in conjunction with background ventilation at low supply flow rate are effective methods for reducing room pollution and exposure to the level that can be achieved with background ventilation alone at much higher supply flow rate. These findings suggest that the implementation of such user-centred ventilation methods can allow the ventilation rate requirements in buildings to be significantly reduced. The results also showed that the integrated exhaust methods provided body cooling to the parts in contact with their surface. The most affected body parts were the back, back side, pelvis, and thighs. It is expected that the local cooling will have a positive effect on thermal comfort in summer seasons and in regions with subtropical or tropical climate conditions. This positive effect must be verified with human subject experiments. The results from the performed energy simulations showed that the use of the ventilated mattress and the ventilated seat cushion offers potential for energy savings. The ventilated mattress in conjunction with background ventilation at 3 air change per hour (ACH) can decrease the annual energy use by 24% to 52% for a double patient room located in a cold climate or hot and humid climate in comparison with conventional mixing ventilation at 4 - 6 ACH. It was found that combining the ventilated seat cushion with mixing ventilation and a chilled ceiling in a call-centre with 14 employees, each using a ventilated seat cushion, reduced the annual energy use by 7 % compared to a system with only mixing ventilation.