A procedure for coupling the heat and mass transfer between inhaled aerosol droplets and the continuous phase that carries the droplets is described and incorporated into a hygroscopic human lung deposition model. Heat and mass transfer coefficients at the airway walls are used to account for the effect of the coupling on wall heat and mass transfer. At 50% ambient RH, 20-degrees-C, and typical inhalation and nebulizer flow rates, by far the largest effect of coupling is on extrathoracic dosages, which are up to 137% and 50% greater than that predicted without coupling for initially isotonic 2.5 mum MMAD (GSD 1.7) and 6.0 mum MMAD (GSD 1.7) aerosols respectively, with 10(6) droplets cm-3. Coupling affects dosages by <10% for all regions only if <25,000 droplets cm-3 are present in the inhaled stream, indicating that coupling between the aerosol droplets and the continuous phase is an important consideration for modelling deposition of most hygroscopic medical aerosols.