Extended lifetime of respiratory droplets in a turbulent vapour puff and its implications on airborne disease transmission

TL;DR

This study uses direct numerical simulations to show that respiratory droplets can remain airborne much longer than previously thought, especially in humid conditions, significantly impacting airborne disease transmission risk.

Contribution

It reveals how turbulent humid respiratory puffs extend the lifetime and range of small droplets, providing new insights into airborne transmission dynamics.

Findings

Small droplets' lifetime extended by over 30 times at 50% humidity

Lifetime increases up to 150 times at 90% humidity

Droplets can travel over two meters within one second

Abstract

To quantify the fate of respiratory droplets under different ambient relative humidities, direct numerical simulations of a typical respiratory event are performed. We found that, because small droplets (with initial diameter of 10um) are swept by turbulent eddies in the expelled humid puff, their lifetime gets extended by a factor of more than 30 times as compared to what is suggested by the classical picture by William F. Wells, for 50% relative humidity. With increasing ambient relative humidity the extension of the lifetimes of the small droplets further increases and goes up to around 150 times for 90% relative humidity, implying more than two meters advection range of the respiratory droplets within one second. Employing Lagrangian statistics, we demonstrate that the turbulent humid respiratory puff engulfs the small droplets, leading to many orders of magnitude increase in their lifetimes, implying that they can be transported much further during the respiratory events than the large ones. Our findings provide the starting points for larger parameter studies and may be instructive for developing strategies on optimizing ventilation and indoor humidity control. Such strategies are key in mitigating the COVID-19 pandemic in the present autumn and upcoming winter.