Relationship between bioaerosol particle size and ventilation removal for airborne infections

Abstract

It is now suggested that COVID-19 can cause airborne infection by fine particles called droplet nuclei and reducing the risk of indoor infection through ventilation is attracting attention as a part of countermeasures against infectious diseases. However, indoor ventilation planning does not take into account the deposition of fine particles on the floor due to gravitational settling or on the wall due to inertia, and thus there is an urgent need to establish appropriate ventilation rate and methods. Therefore, this study aimed to clarify the effect of gravitational settling of suspended particles and ventilation characteristics by natural ventilation using the temperature difference between indoor and outdoor, and evaluated the outflow characteristics and removal efficiency of suspended particles by natural ventilation using CFD analysis. When the outdoor temperature is 5 ºC, particles with a diameter of 80-100 μm are deposited on the floor by gravitational settling in about 20 seconds and are almost completely removed. Particles with diameters of 10 to 70 μm are also deposited by gravitational settling, but some of them are carried by the circulating flow generated in the room by natural ventilation, so the decay of concentration is slow. When the outdoor temperature is 35 ºC, particles with diameters of 30 to 100 μm are almost completely removed from the space in about 30 seconds due to gravitational settling and floor deposition by downward flow generated near the opening. Particles with a diameter of 10 to 20 μm are partially transported by the circulating flow, so that the concentration decay is slow.