We have performed measurements of the hygroscopic response of artificial saliva and deep lung fluid (DLF) using the comparative kinetic electrodynamic balance approach on single aerosol droplets, described in detail in the Supplementary Information. The artificial saliva recipe used (Table ) is composed of a range of ionic components, with sodium chloride and potassium chloride the dominant components, a phosphate buffer, mucin and Dulbecco's modified Eagle's medium (a synthetic cell culture medium that contains numerous components including amino acids, vitamins and glucose). The artificial deep lung fluid recipe used (Table ) is a complex mixture of phospholipids, cholesterol, proteins and antioxidants. Although these artificial recipes do not reflect the biological complexity of real respiratory secretions, the variability between subjects and the possible dependence of composition on disease state, we chose them as broadly representative of the ionic and organic components in human secretions, and to provide some clarity of the dependence of hygroscopic response on chemical composition. We refer to these recipes directly as saliva and deep lung fluid below, although their comparability with real secretions must yet be addressed.