Investigation of air change rate and aerosol behavior during an outbreak of COVID-19 in a geriatric care facility

BackgroundVentilation plays an important role in controlling aerosol transmission of coronavirus disease (COVID-19), and mass transmission of COVID- 19 has been reported in poorly ventilated areas. ObjectiveA real-world mass infection outbreak which occurred in an elderly nursing home, in Miyagi Prefecture, Japan, was simulated experimentally and numerically to investigate the controlling factors and quantify the effectiveness of various natural ventilation settings by means of air change rate (ACR). MethodsUsing the CO2 tracer gas method, the ACR values at the time of the outbreak were estimated to be 2.0-6.2 h-1 in rooms in the facility. Furthermore, a low-cost intervention of opening windows improved the ventilation frequency by a factor of 1.48-5.74. This implies that advective fluid flows are the key in the spread of high CO2 concentration zones. A numerical simulation was performed to obtain spatio-temporal evolution on such high CO2 concentration zones under similar conditions to the present experiment. ResultsThe results showed that ventilation was significantly dependent on the window opening conditions in all rooms (p-values ranging from 0.001 to 0.03 for all the rooms). In contrast, there was no significant dependence on the location of the sensor in any of the areas. Development of high CO2 concentration zones occurs in the first few minutes. Furthermore, the leading edge of such zones towards the common room yields a relatively high fluid velocity, suggesting that the large-scale advective flow dictates the spread of such high CO2 concentration zones. ConclusionsThe present results suggest that secondary infections could occur due to the aerosol advection driven by such large-scale flows, even when the building design adheres the ventilation guidelines. In elderly care facilities, open architectural spaces are recommended to realize quality of life and monitor residents. However, management is required to reduce the downwind infection risk from aerosols and ACR.