Modeling and possible control of droplet-mediated spread of respiratory viruses by the study of the environmental conditions in closed rooms

A clear seasonality has been demonstrated for many respiratory viruses whose transmission relies on droplets released from humans. As a general trend, it was observed that the number of people ill with respiratory viruses is significantly higher in the cold and humid seasons. This phenomenon can certainly be explained by the greater time spent in closed indoor environments, but it also depends on the greater efficiency that the saliva droplets, asvirus carriers, demonstrate in these weather conditions. Basing on the observation that the droplet lifetime depends on many parameters as external temperature (Tair), relative humidity (RH) and air speed, and considering the size distribution of the same droplet, it was modelled the effect of these parameters on the diffusion of respiratory droplets. A lagrangian model of droplet evaporation/settling was used to assess the lifespan of respiratory droplets as a function of Tair, RH and air velocity. In the presence of low Tair and high RH (independently on the air velocity considered), both the distance covered by the droplets and the percentage of viruses were significantly higher than in the presence of medium-high Tair and medium-low RH. These data suggest that it should be possible to modulate these environmental parameters to minimize the spread of virus even in indoor environments.