The contagious power of a drop of saliva

The transmission capacity of a virus is one of the most important factors to take into account in the study of infectious diseases. The vast majority of viruses are transmitted orally. When a person coughs, speaks or sneezes, they exhale a series of particles or drops of saliva with great contagious power into the environment. The evaporation of the drops depends on the different factors that occur in it, and consequently the transmission of the disease varies.

A recent investigation has aimed to study, through computational simulations, the behavior of a saliva particle exposed to different environmental characteristics of a social environment.

The study has been carried out by a team that includes Ainara Ugarte Anero and Unai Fernández Gamiz, researchers from the Department of Nuclear Engineering and Fluid Mechanics of the University of the Basque Country (UPV/EHU), at the Vitoria-Gasteiz School of Engineering.

To study how the saliva droplet behaves through the air, they created a computational fluid dynamics (CFD)-based simulation that examines the state of a saliva droplet moving through the air when a person speaks. , cough or sneeze. “This simulation was carried out in a controlled and simplified environment, that is, instead of analyzing a general sneeze with a series of particles, we focused on the study of a single particle in a closed environment. To do this, we launched droplets of between 0 and 100 microns from a height of about 1.6 meters – about the same distance as the mouth of a human – and the effects of temperature, humidity and size were considered. gout”, explains Unai Fernández Gamiz.

From left to right: Ekaitz Zulueta, Ainara Ugarte-Anero, Oskar Urbina-Garcia and Unai Fernandez-Gamiz. (Photo: Nuria González, UPV/EHU)

The results show that ambient temperature and relative humidity are parameters that significantly affect the evaporation process. Evaporation time tends to be longer when the ambient temperature is lower. And particles with a smaller diameter will evaporate quickly while those with a larger diameter have a harder time. Some large particles, around 100 microns, can remain in the environment between 60-70 seconds and are initially transported a greater distance; so, for example, a person might sneeze in an elevator, get out of the elevator, and the particles are still there. Hence the importance of the two meter safety distance in closed environments in the case of COVID-19. According to what has been studied, it seems that this distance may be reasonable to avoid further contagion in the case of COVID-19”, indicates Ainara Ugarte. To this we must also add the humidity. “In a humid environment, evaporation occurs more slowly, so the risk of contagion is greater since the particles remain in suspension for longer,” he adds.

The research staff of the Department of Nuclear Engineering and Fluid Mechanics of the UPV/EHU agree that “this is a fundamental study, but at the same time vital, since it will allow us to undertake studies of much more complex situations in the future. Until now, with the study of the dynamics of a single drop, we have investigated the foundations of a building”.

The study is entitled “Computational characterization of the behavior of a saliva droplet in a social environment”. And it has been published in the academic journal Nature. (Source: UPV/EHU)