Recent studies have shown that enveloped viruses contained in airborne respiratory droplets lose infectability fastest at intermediate ambient relative humidities Hr. However, the precise physicochemical mechanisms that generate such least-favorable conditions for the virus are not fully understood yet. Studying the evaporation dynamics of respiratory like droplets in air experimentally and analytically, we reveal that at high Hr, the salt dissolved in respiratory drops inhibits their evaporation indefinitely. Conversely, at low Hr the drop evaporates leaving a porous solid residue, inside which virions may remain dormant for long times. We conclude that the optimal relative humidity for minimal infectability should coincide with droplets containing the maximum concentration of salt for longest periods of time.