Large Eddy Simulation of Air Escape through a Hospital Isolation Room Single Hinged Doorway—Validation by Using Tracer Gases and Simulated Smoke Videos

Saarinen, Pekka E.; Kalliomäki, Petri; Tang, Julian W.; Koskela, Hannu

However, despite these precautions, containment failures can happen and the operation of the isolation room doors could be one of the main contributors [20] [21] [22] . A case study by Tang et al. [20] showed that the operation of isolation room doors can lead to containment failure. In the present study, the air migration from the isolation room induced by door-opening and passage has been modelled. Even though the air volume migrated (AVM) is closely connected to the number of pathogens potentially escaping, a linear dependence requires that the pathogens are aerosolized and fully airborne, and therefore capable of long-range transmission. Whilst this is the case for some infectious agents, others are transmitted by larger droplets, subject to gravitational settling. A recent review by Fernstrom and Goldblatt [23] summarizes the research on the aerobiology of infectious aerosols. The division between airborne and droplet transmission is complicated by the fact that in dry air, the droplets desiccate rapidly, leaving so-called droplet nuclei that are airborne. This may happen prior to the droplet/droplet nuclei reaching the floor. Or the droplet nuclei can be resuspended and become airborne again after settling onto a surface. The evaporation of water-containing droplets is significantly reduced in air of high humidity, making them more prone to settling under gravity [24] . According to the definition adopted by WHO, particles with a diameter of less than 5 μm are considered airborne. However, under normal air conditions, particles up to 50 μm dry out completely within 0.5 seconds, and particles up to 100 μm would totally evaporate before falling 2 m [25] . So droplets/particles of these diameters to begin with may quickly evaporate to fall within this socalled 'airborne' range. A human cough contains both droplets and fine, airborne particles that can be simulated by tracer gas [24] . The infectious droplet nuclei generated from human respiratory sources have a diameter 0.5-5 μm with average being less than 1 μm [26, 27] . The estimations about the extent to which airborne transmission contributes to the overall infection rates in hospitals vary from 10% to 30% [23] .