Date Published: November 15, 2011
Publisher: SAGE-Hindawi Access to Research
Author(s): Amir A. Aliabadi, Steven N. Rogak, Karen H. Bartlett, Sheldon I. Green.
Health care facility ventilation design greatly affects disease transmission by aerosols.
The desire to control infection in hospitals and at the same time to reduce their carbon
footprint motivates the use of unconventional solutions for building design and associated control measures. This paper considers indoor sources and types of infectious aerosols, and pathogen viability and infectivity behaviors in response to environmental conditions. Aerosol dispersion, heat and mass transfer, deposition in the respiratory tract, and infection mechanisms are discussed, with an emphasis on experimental and modeling approaches. Key building design parameters are described that include types of ventilation systems (mixing, displacement, natural and hybrid), air exchange rate, temperature and relative humidity, air flow distribution structure, occupancy, engineered disinfection of air (filtration and UV radiation), and architectural programming (source and activity management) for health care facilities. The paper describes major findings and suggests future research needs in methods for ventilation design of health
care facilities to prevent airborne infection risk.
The spread of infectious disease is of global concern for social and economic reasons. For example, seasonal influenza kills 200–500 thousand people annually. In 2009-2010, influenza A (H1N1) caused 17,000 deaths worldwide, many among whom were healthy adults [1, 2]. In 2002-2003, severe acute respiratory syndrome (SARS) killed more than 700 people and spread into 37 countries causing a cost of $18 billion in Asia [2–5]. These recent outbreaks remind us of the potential for a pandemic such as the Spanish flu of 1918–1920 which killed 50–100 million people .
For effective ventilation design of a health care facility, one needs to be able to quantify and predict airborne infection risk. The informed selection of one ventilation design strategy over another requires the use of suitable metrics. To provide a useful prediction, many input parameters need to be supplied to an airborne infection risk model or experiment. The accuracy and extent of these parameters, of course, depend on the model or experiment complexity and the desired level of detail for the expected results. The key factors of the airborne infection process, which determine the organization of our discussion, are present in the Wells-Riley risk model for a well-mixed room 
Past outbreaks of disease transmission in health care facilities have identified many pathogens such as Tuberculosis, influenza, and Aspergillosis, whose spread is strongly linked to airborne transmission. As a result, there is renewed interest in prevention of airborne disease transmission in health care facilities. Research has led to the development of useful methods for the prediction of various aspects of airborne disease transmission. Physical modeling of, and experiments on, aerosol transport have revealed mechanisms for dispersion, heat, and mass transfer of aerosols in different size regimes from submicrometer to millimeter. Many detailed infection models such as Wells-Riley model (1) and dose-response model (e.g., (5) to (9)) have been suggested for prediction of the spread of airborne disease in health care facilities. The current state of research has also shown that important sources for airborne pathogens including human activity (expiratory, resuspension, and shedding) and building microbial ecology (indoor environment and HVAC components). The viability and infectivity response of various airborne pathogens to environmental conditions, such as temperature and relative humidity, have also been understood and quantified in considerable depth. For example, animal tests, culture methods, molecular methods, and plaque assay methods have been used to measure pathogen viability and infectivity decay rates for many different classes of microbes although all these techniques suffer from substantial shortcomings. The effect of different ventilation strategies (mixing, displacement, and natural) on air exchange rates and the spread of aerosols has also been studied. For example, mixing type ventilation can achieve sufficient dilution of contaminated air to reduce the infection rate. A displacement ventilation strategy, if implemented successfully, helps segregate contaminated from noncontaminated air, thus reducing the infection risk. Under ideal conditions, natural ventilation has been shown to exhibit very high air exchange rates (and thus dilution) that reduce the airborne infection risk. In addition, various building codes have implemented preventive measures to reduce the airborne infection risk. For example, high efficiency filtration and high air exchange rates are mandated for critical care units (surgery and protective environment rooms), and exhaust outlets are recommended to be installed close to contaminant sources.