To make the physics of person-to-person virus transmission from emitted droplets of oral fluid while speaking easily understood, we present simple and transparent algebraic equations that capture the essential physics of the problem. Calculations with these equations provide a straightforward way of determining whether emitted droplets remain airborne or rapidly fall to the ground, after accounting for the decrease in droplet size from water evaporation. At a relative humidity of 50%, for example, droplets with initial radii larger than about 50 μm rapidly fall to the ground, while smaller, potentially virus-containing droplets shrink in size from water evaporation and remain airborne for many minutes. Estimates of airborne virion emission rates while speaking strongly support the proposal that mouth coverings can help contain the COVID-19 pandemic.

为了使人与人之间从口液中散发出来的人间病毒传播的物理原理易于理解，我们提出了简单透明的代数方程式，以捕捉问题的基本物理原理。使用这些方程式进行的计算提供了一种直接的方法，可以确定排放的液滴是保留在空气中还是迅速下落到地面，这是考虑到水蒸发造成的液滴尺寸减小后的结果。例如，在相对湿度为50％时，初始半径大于约50μm的液滴会迅速掉落到地面，而较小的，可能包含病毒的液滴会因水蒸发而缩小尺寸，并在空气中停留数分钟。