Theoretical Prediction of the Size and Lifetime of Evaporating Sneeze Droplets in a Confined Space: A Guideline to Control of COVID-19 Virus Transmission

The size and lifetime of evaporating sneeze droplets in the indoor environment were studied experimentally and theoretically. The effects of indoor temperature T∞ and indoor humidity RH∞ on evaporating droplets with the initial diameters of 4.9, 8.1, 17.2, and 29.7 μm were investigated. The size distribution and mean size of droplets were obtained by a laser particle sizer. The experimental data showed that the possibility of aerosolized droplets increased from 25.5 to 36.1 % by increasing T∞ from 18 to 30 °C and decreased from 36.1 to 13.6 % by increasing RH∞ from 30 to 60 %. A one-dimensional droplet evaporation model was used to estimate the lifetime of the droplet. A critical RH∞ of 40 % was found; above it, the lifetime of the droplet exponentially increases. The effect of the initial diameter of droplets was higher than that of RH∞ and also the impact of RH∞ was higher than that of T∞ on the lifetime of the aerosolized droplet nuclei. A significant effect of environmental conditions on the lifetime of the droplet was found over the range of 26 °C ≤ T∞ ≤ 30 °C and RH∞ ≤ 40 %, while the effect decreased in the range of 18 °C ≤ T∞ ≤ 22 °C and RH∞ 40 %, where a minimal shrinkage of droplets took place because of the hygroscopic growth of droplets. The results of this study do not imply that the COVID-19 virus will be deactivated at the end of the lifetime of the droplet, but it represents that controlling the indoor environment is important for droplets to carry the virus.