Airborne lifetime of respiratory droplets

We formulate a model for the dynamics of respiratory droplets and use it to study their airborne lifetime in turbulent air representative of indoor settings. This lifetime is a common metric to assess the risk of respiratory transmission of infectious diseases, with a longer lifetime correlating with higher risk. We consider a simple momentum balance to calculate the droplets' spread, accounting for their size evolution as they undergo vaporization via mass and energy balances. The model shows how an increase in the relative humidity leads to higher droplet settling velocity, which shortens the lifetime of droplets and can, therefore, reduce the risk of transmission. Emulating indoor air turbulence using a stochastic process, we numerically calculate probability distributions for the lifetime of droplets, showing how an increase in the air turbulent velocity significantly enhances the range of lifetimes. The distributions reveal non-negligible probabilities for very long lifetimes, which potentially increase the risk of transmission. (C) 2022 Author(s).

Automated summary

We propose a model for the dynamics of respiratory droplets and investigate their airborne lifetime in turbulent air typical of indoor environments. The droplet lifetime is an important metric for assessing the risk of respiratory transmission of infectious diseases. By considering a simple momentum balance and accounting for the evaporation of droplets, we show that an increase in relative humidity leads to higher settling velocity and shorter droplet lifetime, potentially reducing the transmission risk. Using a stochastic process to emulate indoor air turbulence, we numerically calculate probability distributions for droplet lifetime and find that increased turbulent velocity significantly extends the range of lifetimes, including non-negligible probabilities for very long lifetimes that may heighten the transmission risk.