Assessment of risks and control of airborne transmission of infectious diseases in high-rise residential environments

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Assessment of risks and control of airborne transmission of infectious diseases in high-rise residential environments

 

Author: Wu, Yan
Title: Assessment of risks and control of airborne transmission of infectious diseases in high-rise residential environments
Degree: Ph.D.
Year: 2017
Subject: Natural ventilation.
Dwellings -- Heating and ventilation.
Airborne infection -- Prevention.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxiv, 189 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2949919
URI: http://theses.lib.polyu.edu.hk/handle/200/8833
Abstract: Natural ventilation, as an energy-efficient ventilation strategy, may negatively affect public health because of the pollutant dispersion caused by air movement. Airborne transmission may be responsible for the spread of various respiratory infectious diseases, such as tuberculosis, measles, influenza, smallpox, and severe acute respiratory syndrome (SARS). Identifying the possible airborne transmission routes and assessing infection risks are essential for controlling infectious diseases. The SARS outbreak in 2003, in which 321 cases were identified in the Amoy Gardens estate, implies that both "inter-unit dispersion" and "inter-building dispersion" should not be overlooked, especially in densely populated residential districts. The inter-unit dispersion induced by single-sided natural ventilation has been systematically investigated in previous studies. The current study focuses on the inter-unit dispersion induced by air infiltration or cross-ventilation and the near-field inter-building dispersion in high-rise residential (HRR) environments. On-site measurements and multi-zone modeling were conducted in slab-type and cross-type buildings, respectively, to assess inter-unit dispersion with cross-ventilation or air infiltration. Numerical studies were carried out to analyze inter-building dispersion. The unsteady Reynolds-Averaged Navier-Stokes (URANS) and detached eddy simulation (DES) models were compared, and a reliable computational fluid dynamics method was developed. A possible control strategy for inter-unit dispersion with single-sided natural ventilation was also estimated. Infectious risk model by Wells-Riley was used. The infectious risk of inter-unit dispersion driven by air infiltration is higher than that of inter-unit dispersion driven by cross-ventilation. In the on-site measurement, the maximum cross-infection risk estimated by the Wells-Riley model is 9% for the dispersion driven by air infiltration and 5% for that driven by cross-ventilation. The cross-infection risk through air infiltration route with the value of 9% is even higher than that via the vertical spread route through single-sided open windows with the value of 6.6%. The tracer gas concentrations in the receptor rooms are one order lower than the concentration in the index room, and the relative risks are also one order lower in terms of infection probability. The assessed cross-infection risk in the multi-zone modeling can be higher than 20%, which is significantly higher than 9% in the on-site measurements. These results are due to that the predicted air infiltration rate in the multi-zone modeling is below 0.7 air change rate per hour (ACH), while the rate is 3-4.5 ACH in the on-site measurements because of the leaky building condition. This parametric analysis reveals that cross-infection risk is significantly affected by air infiltration rate, which would be much higher at lower air infiltration rates.
Properly improving the tightness of internal doors or windows and encouraging the openness of external windows can be beneficial to the control of inter-unit dispersion induced by air infiltration. Practically, in residential building design, the internal windows should be avoided. Moreover, the airtightness of individual entrance doors to public spaces, such as corridors, staircases, and elevators, should be improved. These strategies conform to the concept of urbanization construction, which is beneficial for the fire control in HHR buildings and meets the privacy requirements for modern living. As far as occupant behavior and public health are concerned, opening external windows should be encouraged to increase the amount of fresh air. However, this approach may elevate the cross-infection risk with single-sided natural ventilation. Mechanical exhaust (ME) is a control method for cross-household infections induced by single-sided natural ventilation in HRR buildings. An ME airflow rate threshold exists, above which the two-way airflow of single-sided natural ventilation can transform into a one-way inflow. But, some complexities are found, such as the ME-on only in the source room scenario may result in more re-entry of the outflow to the upper floor. Therefore, opening household exhaust fans centrally is recommended to reduce the re-entrance of outflow during epidemic outbreaks. Installing a central ME system in the ventilation design of residential buildings is a better approach. This will be the development trend of urbanization, and can be crucial for controlling infectious diseases and improving public health. The cross-infection risk of inter-building dispersion should not be underestimated either. The concentrations of pathogens near the downstream buildings are only one order lower than the concentration in the source building. The DES model was found to better reproduce the unsteady fluctuations of airflow around a group of buildings than the URANS model. Disregarding the effects of the surrounding buildings can significantly overestimate the downward dispersion and the risk levels on lower floors. The present study investigates the airborne transmission routes and mechanisms in HRR environments and assesses the cross-infection risk. Recommendations with regard to both building ventilation designs and occupant behaviors are stated. The findings of the study have practical implications for public health in urbanization.

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