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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 2, February 2019.
By Jinkyun Cho, Ph.d., Associate Member ASHRAE; Kyunghun Woo; Byungseon S. Kim, Ph.D., Member ASHRAE
About the Authors
Jinkyun Cho, Ph.D, is senior research engineer at KCL (Korea Conformity Laboratories), Seoul, South Korea. Kyunghun Woo is senior researcher at Samsung C&T Corporation, Seongnam, South Korea. Byungseon S. Kim, Ph.D., is professor at Yonsei University, Seoul, South Korea.
The study discussed in this article evaluated the ventilation performance of three strategies for HVAC control for airborne infectious diseases induced by contaminated exhaled air from patients in an airborne infectious isolation room (AIIR). This article examines airflow path and airborne pollutant distribution by computational fluid dynamics modeling and field measurement. In hospitals, the risk of airborne virus diffusion mainly depends on airflow behavior and changes in direction caused by supply air and exhaust air locations. An improved isolation room ventilation strategy has been developed, and is found to be the most efficient in removing contaminants based on observations and simulation results from three ventilation systems.
Introduction
Airborne transmission is one of the main spread routes for a number of infectious diseases such as smallpox and tuberculosis. More than 8,000 reported cases of Severe Acute Respiratory Syndrome (SARS) resulted in 774 deaths and led to a wave of research and standardization of medical facilities with respect to airborne diseases. 36 patients died and 186 people were infected during the outbreak of Middle East Respiratory Syndrome (MERS) in South Korea in May 2015. Outbreaks of such coronaviruses (airborne diseases) in hospitals elevates the risk of infection from patients to health-care workers (HCWs) and other patients. Mainly because of poor ventilation and ineffective disinfection in one hospital, MERS viruses began to spread rapidly to patients, visitors, and even to HCWs. The plan of an AIIR with negative pressure includes a complex process of decisions. The specifications of mechanical ventilation system, location, layout, interior finishing, and AIIR facilities are critical to the design concepts. Because of exhaust air (EA) and supply air (SA) locations, the risk of virus dispersal at the hospital is influenced by changes in movement and direction of airflow.
Negative Pressure AIIR Design
As shown in Table 1, a negative pressure AIIR design requirement varies from country to country. AIIRs are required to be designed the single-pass approach to bring clean air from the clean zone to the contaminated zone. According to ASHRAE/ASHE Standard 170-2017, Ventilation of Health Care Facilities, the pressure difference required to maintain negative pressure is minimum 2.5 Pa. The actual negative pressure level will depend on several factors: differences in the SA and EA volume; airflow paths; and airflow openings and physical configuration of the acute care patient room. To maintain negative pressure in a room, the EA volume needs to be 10% larger than the SA volume. For a room with low airtightness, the HVAC system may not be able to provide the necessary EA/SA airflow differentials. AIIRs in existing health-care facilities needs to achieve at least 6 air changes per hour (ACH) in order to reduce the concentration of pollutants.
The problem of AIIR ventilation systems can include air mixing and inappropriate directional airflow pattern. Ideally, the clean SA should be introduced near HCWs and then, EA should be removed near the patients.12 The typical AIIR ventilation strategy employs a ceiling SA and ceiling EA system and/or optional air recirculation unit with HEPA filter such as a fan filter unit (FFU).
Table 1
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