This study reports on a model for the evaporation of mono-disperse water droplets into air that can be used to aid the design of nebuliser based humidification systems for respiratory treatments and also for predicting droplet size distributions for pulmonary drug delivery devices under either isothermal or adiabatic conditions. The governing equations are developed from the coupled mass and energy balances for the droplets and for the surrounding air. An adjustment to one of these equations allows for the modelling of either isothermal or adiabatic conditions. Additionally, the proposed model allows for the variation of fluid transport and thermodynamic properties during the solution of the governing equations. The model is compared to and validated against an overall mass and energy balance and the literature. Confirmation of the general evaporation characteristics of water droplets evaporating into air is gained from the model results and, its accuracy, usability and applicability are improved over existing models. The proposed model indicates that the droplet evaporation process in humidification devices is largely unaffected by the initial water temperature and also indicates advantages of adiabatic conditions when using cascade impactor technology for droplet sizing. (C) 2013 Elsevier Ltd. All rights reserved.