Droplet Fate in a Cough Puff

The dynamic and thermodynamic evolution of droplets, in a size range characterizing a cough, has been analysed using basic equations of motion and coupled to the evolution of a spherical cloud puff in which they are supposed to be expired. It has been found that the maximum contamination range of the emitted droplets is controlled by two different mechanisms: surface evaporation and inertia-gravitational settling--with a switching threshold between them for a radius around a few tens of microns. For the smallest droplets, the environmental conditions (the temperature and humidity) are found to be very effective in determining the contamination range, even for weak entrainment in the cloud puff. This last fact could be of some relevance in the seasonal behaviour of air-borne epidemics