A large number of infectious diseases are 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.

大量传染病是通过呼吸道飞沫传播的。这些飞沫在空气中停留的时间有多长，它们能传播多远，以及它们可能携带的病原体存活多长时间，都是飞沫传播疾病传播的决定性因素。该主题极其多方面，涉及不同学科，但其中大多数在物理学界很少得到考虑。在这篇综述中，我们讨论了控制呼吸道飞沫和其中捕获的任何病毒命运的物理原理，重点是相对湿度的作用。重要的是，较低的相对湿度（例如冬季室内和飞机内的情况）会促进蒸发，甚至使最初的大液滴以气溶胶的形式长时间悬浮在空气中。更重要的是，相对湿度通过空气-水界面的风化和灭活等多种物理机制影响气溶胶中病毒的稳定性，但人们对其在病毒灭活中的作用仍知之甚少。阐明相对湿度在疾病飞沫传播中的作用将使我们能够设计有助于减少传播机会的预防措施，特别是在室内环境中。