The Future Trend In Building Design Towards Minimizing Transmission Of Airborne Infectious Diseases

Prof. Chun Chen, Assistant Professor - Department of Mechanical & Automation Engineering, The Chinese University of Hong KongHolding a Ph.D. degree from the School of Mechanical Engineering, Purdue University, Prof. Chun has profound knowledge of the engineering industry, with deep interests in the areas of indoor air quality, energy-efficient buildings, aerosol dynamics, and airborne infectious disease transmission.

The transmission of airborne infectious diseases, such as avian flu, influenza, tuberculosis and severe acute respiratory syndrome (SARS), has become a global public health issue. For example, the Spanish flu between 1918 & 1919 infected about one-quarter of the global population and killed more than 40 million people. Every year, influenza epidemics cause about 47,200 deaths in the U.S. In an estimation by The World Bank, a pandemic of avian flu among humans could cost the global economy $800 billion a year. The tuberculosis infections found in 22 countries have caused 1.87 million deaths. During the SARS outbreak, there were more than 8,000 people infected globally. These airborne infectious diseases clearly have caused endless social and economic disruptions. Thus, it is crucial to understand and control the transmission of airborne infectious diseases in order to reduce their influence on human health.

In recent decades, many outbreaks of airborne infectious diseases have occurred in buildings where people spend most of their time. An infected person can exhale droplets containing infectious viruses through breathing, coughing or sneezing. These airborne droplets can be transported to the breathing zones of other persons via the air in the buildings. If the susceptible individuals inhale these droplets, cross infection of the disease may occur. These transmission routes indicate that the air distribution in buildings can play a significant role in the transport of airborne exhaled droplets. If the airflow can directly remove the exhaled droplets from the room or block the transmission route to the breathing zones of other occupants, the infection risks can be minimized. Therefore, it is crucial to carefully design the airflow distribution in buildings in order to reduce the airborne infectious disease transmission.

The most conventional air distribution mode in buildings is that of mixing ventilation. Both the supply air diffusers and exhausts are usually installed at the ceiling level. The conditioned air with fresh air is supplied to the room. Due to the strong supply air jets, the air is well
mixed in the room so that the fresh air can reach to the whole space. However, as the air in the room is well mixed, when an infected person exhales droplets, these droplets can be transported to the whole space including the other occupants' breathing zones. In such cases, even if the distance between the infected person and a potential receptor is long, the exhaled infectious droplets can still be inhaled by the receptor and cause cross infections. Thus, the person-to-person airborne infectious disease transmission in buildings with mixing ventilation can be rather significant.

Through the computer simulations, the effectiveness of the air distribution system on controlling airborne infectious diseases transmission can be correctly evaluated to assist the building design

Current Trend of Building Ventilation Design
To reduce the airborne infectious disease transmission in buildings, researchers proposed several advanced ventilation design systems. For example, displacement ventilation was originally proposed for reducing energy consumption for cooling. Displacement supply air diffusers are installed at the lower level of the walls, while the exhausts are installed at the ceiling level. Cool air is supplied to the room at a low speed. As the cool air tends to stay at the lower level of the room, a layer of cool air is formed. When the cool air encounters a heat source, such as a person, the air is heated and moves upwards. The upward airflow goes through the heat source and brings the heat to the exhausts. This is also known as the buoyancy effect. The heat removal efficiency is significantly improved in this way. From the perspective of removing exhaled infectious droplets, the displacement ventilation system is also advantageous as the fresh air also goes upwards with the buoyancy effect, so that the exhaled droplets can be directly brought to the exhausts without traveling to other occupants' breathing zones. Therefore, the cross infections due to airborne transmission can be minimized.

The traditional approach to achieve the optimal design of air distribution in buildings is to conduct small or full-scale experiments in a simulated environment or with trial-and-error tests. The whole process may take a few months and cost a lot of money and resources. With the rapid development of advanced computer modeling techniques, the design can be completed more efficiently. Currently, the most popular and powerful computer aided design tool for air distribution in buildings is Computational Fluid Dynamics (CFD). To obtain the information on airflow distribution, CFD numerically solves a set of partial differential equations for the conservation of mass, momentum (Navier-Stokes equations), energy and turbulence quantities. The solution includes the distributions of air velocity, pressure, temperature and turbulence parameters. Based on the calculated airflow, we can continue to solve the droplet equations to obtain information about person-to-person infectious droplet transport in buildings. Through the computer simulations, the effectiveness of the air distribution sys-tem on controlling airborne infectious diseases transmission can be correctly evaluated to assist the building design.

The Way Forward in Building Design
The inverse design technique using genetic algorithm and adjoint method, combined with CFD has been developed in recent years to directly achieve the optimal design of air distribution systems that minimize the air-borne infectious diseases transmission in buildings. In comparison with the traditional trial-and-error experiment method, applying the inverse design approach with CFD in building design can reduce design time down to a tenth, depending on the computing resources available. The users only need to install the relevant inverse design software and learn how to operate it. As the initial set-up costs of adapting to inverse design are not high, it is expected such advanced technologies will be widely used by the industry practitioners in designing green and healthy buildings in the long run.