Based on the observation of prompt crystallisation in the single droplet drying measurements, we can assume that evaporating respiratory fluid droplets undergo a phase change to a particle of non-spherical shape once they reach a water activity of 0.45. The particles could be crystalline, amorphous or mixed phase, and it is not possible from our measurements to discriminate between these. Although there may be some inhomogeneity in the concentration profile of evaporating droplets and the surface will increase in solute concentration more rapidly than the core, we do not include this in our modelling. In Fig we indicate the combinations of starting droplet size and environmental RH that lead to a phase change during the drying process before sedimentation. For all droplets <~100 m diameter, the phase change occurs prior to deposition provided the RH is below the reported phase change RH. For droplets >100 m diameter, phase change occurs before sedimentation only when the drying rate is large enough that the solute concentration can rise sufficiently rapidly for the phase change to occur. Droplets >120 m diameter are unlikely to undergo a phase change before deposition, but they may be strongly supersaturated in salt and will likely form salt crystals on impact on a surface.