in the limit where surface curvature does not play a significant role in determining droplet vapour pressure and the hygroscopic response can be represented by a single number defining the shape of the hygroscopic growth curve. It should be noted that this single-parameter treatment of hygroscopic growth is increasingly in error as the RH decreases, as is apparent in Figure when comparison is made with the comprehensive E-AIM model for aqueous sodium chloride droplets.  values of 0.290 ± 0.01 and 0.208 ± 0.008 were measured for artificial saliva and deep lung fluid, respectively, from the growth factors at 95% RH. As an indicator of degree of hygroscopicity, a value of 1.2 is commonly reported for NaCl solution droplets, and a value of <0.05 represents an organic aerosol of low hygroscopic growth. It is clear from the model presented in Figure that the saliva and deep lung fluid are considerably less hygroscopic than salt solution, with values of GFd of 1.089 ± 0.003, 1.065 ± 0.003 and 1.301 at 50% RH, respectively. Thus, if all droplets were to start at 100 m at 99.5% RH, they would equilibrate to droplets of 28.0, 30.4 and 20.9 m diameter at 50 % RH, respectively. Although saliva and deep lung fluid both contain significant mass fractions of various salts (see Table ), they are also rich in organic components and retain less water at all RH values than the corresponding pure salt solution. This is reflected in the lower reported value for the -parameter for saliva and deep lung fluid relative to sodium chloride. Interestingly, despite the chemical complexity of saliva and deep lung fluid, the similar fractions of organic and salt components result in closely matching hygroscopicity curves. Functional fits for the equilibrium solution compositions over the full RH range are shown in Fig and are given in Table . These fits allow the equilibrium composition, and corresponding density, to be calculated down to an RH at which a phase change is observed. The mass fraction of solute (MFS) tends to 1 only at 0% RH. Indeed, the possibility that phase change occurs can be ignored in order to estimate the compositional dependence of the respiratory aerosol down to fully dry conditions. At the phase change RH, it is possible that the particle may undergo full crystallisation or partial crystallisation, and the RH of this phase change is very close to the accepted RH for crystallisation of NaCl;