It could be anticipated that the high contents of mucin and surfactants in respiratory fluid droplets would lead to delayed moisture release with the droplets becoming viscous on drying or a surface film delaying water evaporation. We have seen such behaviour for the drying kinetics of a wide range of high molecular weight systems including saccharides and tri-block polymers, and the impact of this on atmospheric aerosols has long been debated. If the viscosity of a droplet rises during drying, the diffusion constant of evaporating water molecules decreases imposing a kinetic constraint on the moisture release kinetics. We have also demonstrated that the formation of solid condensed monolayer films on the surface of an evaporating droplet impedes water evaporation. In this work we observed a small degree of kinetic limitation for deep lung fluid, as shown by the delay to reach equilibrium at 50 and 65% RH in Fig , but no such limitation for saliva (Fig ), suggesting that the diffusional timescale of water in the drying particles is typically shorter than the drying time (of order 1s). This conclusion is consistent with a value for the viscosity that remains below 1 Pa s, the viscosity of a relatively viscous liquid (e.g. glycerol), three orders of magnitude more viscous than water.