Are Runners More Prone to Become Infected with COVID-19? An Approach from the Raindrop Collisional Model

It is known that COVID-19 spread mainly from person-to-person through respiratory droplets produced when an infected person coughs or sneezes, and as a result certain ideas about contagious of COVID-19 have been spread. One of them is the widespread belief that close runners, owing to the stronger exhalation, can be more prone to be infected with COVID-19 because the collision with the suspended respiratory droplets should the runner in front be infected. However, because of the low Stokes number this idea cannot be generalized without carefully thought and in fact can be put into question. Utilizing the raindrop collisional model and with the help of computational fluid dynamics (CFD), it is shown that the probability of collision with respiratory droplets is not always increasing with the approaching velocity of the runner but rather there is a maximum velocity threshold at which the efficiency of collision drops.