Despite the conventional delineation between droplets (particles >5 m diameter) and respirable aerosols (<5 m), the exact picture is more complex and this arbitrary threshold has been widely challenged. Most recently, Prather et al. have suggested that a more appropriate delineation between droplet and aerosols would be at 100 m, a size that more appropriately reflects a change in aerodynamic behaviour, the potential for inhalation and the efficacy of non-pharmaceutical interventions such as physical distancing. Although approximately bimodal, the size distributions of small respiratory particles (mass median aerodynamic diameter, MMAD, of ~1-2 m) and large droplets (MMAD 100 m) are continuous with particles spanning all sizes. Indeed, we now know that over 99% of particles from an expiratory event are of respirable size already, dominating over the number of any aerosol nuclei formed from the evaporation of large droplets. The relative humidity (RH) in the mouth and upper respiratory tract can be very close to 100% although this may depend on disease state, and so droplets and aerosols begin their transport from the infected individual with high moisture content. Previous studies have suggested that particles can reduce in size by more than a factor of 2, dependent on the ambient RH and the moisture content of the particles on exhalation. The buoyancy of the turbulent warm air cloud can lead to transport over much larger distances than previously anticipated for large droplets, and the air flow patterns in the room can lead to enhancements in the lifetime of suspended particles. Although our understanding of the transport of exhaled particles is advancing rapidly, there remain significant gaps in our knowledge of the droplet microphysics occurring during drying and sedimentation. In particular, the evaporation kinetics models are based on an assumption that the droplets are pure water or salt solution, there is little recognition that droplets could change phase during drying to form crystalline particles, and the interplay of ambient RH, drying rate and moisture content on the final dried particle morphology is often not considered, particularly for droplets that contain a large fraction of mucins, high molecular weight, heavily glycosylated proteins. Not only could these microphysical properties impact on the transported distance of droplets on exhalation through impacting on aerodynamic size, but also they could impact on survival of viruses while airborne through influencing moisture content and particle phase. We address these uncertainties here.