Relative humidity in droplet and airborne transmission of disease

TL;DR

This review explores how relative humidity influences the behavior and stability of respiratory droplets and viruses, impacting disease transmission and informing preventive indoor health measures.

Contribution

It provides a multidisciplinary overview of the physical mechanisms by which humidity affects droplet persistence and virus stability, highlighting gaps in current understanding.

Findings

Low humidity prolongs aerosol suspension of droplets.

Humidity affects virus stability through physical mechanisms.

Understanding humidity's role can improve disease prevention strategies.

Abstract

A large number of infectious diseases is transmitted by respiratory droplets. How long these droplets persist in the air, how far they can travel, and how long the pathogens they might carry survive are all decisive factors for the spread of droplet-borne diseases. The subject is extremely multifaceted and its aspects range across different disciplines, yet most of them have only seldom been considered in the physics community. In this review, we discuss the physical principles that govern the fate of respiratory droplets and any viruses trapped inside them, with a focus on the role of relative humidity. Importantly, low relative humidity -- as encountered, for instance, indoors during winter and inside aircraft -- facilitates evaporation and keeps even initially large droplets suspended in air as aerosol for extended periods of time. What is more, relative humidity affects the stability of viruses in aerosol through several physical mechanisms such as efflorescence and inactivation at the air-water interface, whose role in virus inactivation nonetheless remains poorly understood. Elucidating the role of relative humidity in the droplet spread of disease would permit us to design preventive measures that could aid in reducing the chance of transmission, particularly in indoor environment.