**Abstract**

The research community agrees that the main indirect way the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spreads among people who do not keep social distance is through the emission of infected respiratory droplets. Infected people exhale droplets of different sizes and emission velocities while breathing, talking, sneezing, or coughing. Complex two-phase flow modeling considering evaporation and condensation phenomena describes droplets' trajectories under the specific thermofluid dynamic boundary conditions, including air temperature, relative humidity, and velocity. However, public health organizations simply suggest a safe distance in the range of 1–2 m regardless of the effect of boundary conditions on droplets' motion. This chapter aims to highlight open research questions to be addressed and clarify how framework conditions can influence safe distance in an indoor environment and which technical countermeasures (such as face masks wearing or heating, ventilation, and air conditioning (HVAC) control) can be adopted to minimize the infection risk.

**Keywords:** SARS-CoV-2, droplets, airborne, aerosol, face masks, HVAC system, safe distance, contagion prevention
