Ventilation characteristics and infection risk in Japanese hospital wards: An assessment based on field measurements and CFD

During the global spread of COVID-19, numerous outbreaks occurred in wards. To evaluate ward ventilation performance and elucidate actual operating conditions, ventilation-characterization surveys, particle-dispersion simulations, and infection-risk assessments were conducted for two wards in Japan. First, field measurements quantified ventilation airflow rates and identified, for each room, air changes per hour (ACH; based on terminal airflow) and ventilation pathways. Second, using a RANS-based model capable of reproducing the dispersion of particles with a size distribution, four cases that combined airflow conditions (design and measured) and door states (open and closed) were simulated. Under the assumption of a single index case, the probability of infection was calculated and the expected number of secondary cases was estimated for a 24-h exposure. Discrepancies between design and measured airflow rates were confirmed in many rooms, and numerous rooms fell below the reference value (2.0 ACH). In the ward that had experienced multiple COVID-19 outbreaks, the four-bed rooms had no exhaust by design; with supply only, they constituted spaces with a higher risk of the spread of infection between rooms. Closing doors suppressed particle outflow to the corridor and inflow into patient rooms, markedly reducing the risk of the spread of infection between rooms. Conversely, when ventilation is insufficient, door closure may worsen indoor air quality. Accordingly, maintaining airflow equivalent to design values is required, and regular inspection, verification and maintenance are recommended. The findings provide a basis for improving the design and maintenance of HVAC systems and for planning infection control measures in wards.