Infectiousness model of expelled droplets exposed to ultraviolet germicidal irradiation coupled with evaporation

Abstract

Respiratory diseases propagated by droplet-based transmission are a serious public health hazard, leading, in extreme cases, to pandemics such as influenza and the recent Coronavirus outbreak. Droplet infectiousness evolves in response to the environment in such a way that it decreases with time and distance from the source. Droplets can also be treated with ultraviolet germicidal irradiation (UVGI) to accelerate the reduction in their survival rate. In addition, airflow management and furnishings in public spaces can be optimized to reduce exposure to expelled droplets. This introduces a role for engineered medical interventions based on precise modelling of the time evolution of droplet infectiousness. However, information is lacking on computational fluid dynamics (CFD) simulations of the time evolution of droplet infectiousness when exposed to both evaporation and UVGI at the same time. Therefore, in this study, we developed and presented algorithms for tracking droplet infectiousness in a CFD simulation. The variations of droplets’ infectiousness were investigated through the combination of parameters describing humidity and temperature, as well as the deployment of UVGI in confined public space environments. We have shown that, for airborne droplets, increased temperature leads to decreased infectiousness in propagation lengths and entrainment time, while increased humidity leads to increased infectiousness propagation lengths and entrainment times. Smaller droplets with diameters ≤ 110 μm remain entrained longer in the air, whereas droplets with a diameter ≥ 140 μm travel in the air stream for a relatively short time before falling to a surface due to gravity, depending on the specific relative humidity (RH) and temperature conditions. Thus, our data suggest that infectiousness of droplets is substantially reduced due to the influence of evaporation, and it is further decreased when exposed to UV irradiance. We also show that, due to the combined influence of UVGI and evaporation conditions, the infectiousness of droplets decreased faster. Our model can track droplet infectiousness, thus helping to understand how the spread of infectious droplets is reduced in a confined environment.