ASHRAE ST-16-004

Abstract

Scope: Exposure to airborne influenza (or flu) from a patient’s cough and exhaled air causes potential flu virus transmission to the persons located nearby. Hospital-acquired influenza is a major airborne disease that occurs to health care workers (HCW). This paper examines the airflow patterns and influenzainfected cough aerosol transport behavior in a ceiling-ventilated mock airborne infection isolation room (AIIR) and its effectiveness in mitigating HCW’s exposure to airborne infection. The computational fluid dynamics (CFD) analysis of the airflow patterns and the flu virus dispersal behavior in a mock AIIR is conducted using the room geometries and layout (room dimensions, bathroom dimensions and details, placement of vents and furniture), ventilation parameters (flow rates at the inlet and outlet vents, diffuser design, thermal sources, etc.), and pressurization corresponding to that of a traditional ceiling- mounted ventilation arrangement observed in existing hospitals. The measured data shows that ventilation rates for the AIIR are about 12 air changes per hour (ach). However, the numerical results reveals incomplete air mixing and that not all of the room air is changed 12 times per hour. Two life-sized breathing human models are used to simulate a source patient and a receiving HCW. A patient cough cycle is introduced into the simulation and the airborne infection dispersal is tracked in time using a multiphase flow simulation approach the effect of air filtration and prolong the presence of flucontaminated air at the HCW’s zone. Immediately after the patient coughs (0.51 s), the cough velocity from the patient’s mouth drives the cough aerosols toward the HCW standing next to patient’s bed. Within 0.7 s, theHCWis at risk of acquiring the infectious influenza disease, as a portion of these aerosols are inhaled by the HCW. As time progresses (5 s), the aerosols eventually spread throughout the entire room, as they are carried by the AIIR airflow patterns. Subsequently, a portion of these aerosols are removed by the exhaust ventilation. However, the remaining coughaerosols reenterandrecirculate in theHCW’szone until they are removed by the exhaust ventilation. The infectious aerosols become diluted in the HCW’s region over a period of 10 s because of the fresh air supplied into the HCW’s zone. The overall duration of influenza infection in the room (until the aerosol count is reduced to less than 0.16%of the totalnumberof aerosols ejected from the patient’s mouth) is recorded as approximately 20 s. With successive coughing events, a near-continuous exposure would be possible. Hence, the ceiling-ventilation arrangement of the mock AIIR creats an unfavorable environment to theHCWthroughout his stay in the room, and the modeled AIIR ventilation is not effective in protecting theHCWfrominfectiouscoughaerosols. The CFD results suggest that the AIIR ceiling ventilation arrangement has a significant role in influencing the flu virus transmission to the HCW