Cascade impaction is an important tool for measuring aerosol distributions from wet nebulizers; however, results vary depending on laboratory and technique. The focus of this study was to reconcile the contribution of particle evaporation to these reported differences. To measure the effect of evaporation, we compared aerosol distributions from circuits ventilated with humidified air, ambient air, and a nonventilated, standing cloud circuit using low-flow cascade impaction (1.0 L/min). Aerosol distributions were similar for the humidified/ventilated and standing cloud models [mass median aerodynamic diameter (MMAD) 3.4 microm, and 3.6 microm Aero-Eclipse, 5.8 and 5.1 microm Misty-Neb, 3.8 and 3.2 microm Pari LC Plus]. In the ventilated/ambient air model, smaller particle sizes were measured (2.2 microm AeroEclipse, 2.4 microm Misty-Neb, 2.1 microm Pari LC Plus). Techniques of cascade impaction significantly affected measured aerosol distributions. MMAD were defined by nebulizer type and conditions of particle evaporation not by impactor. Aerosol mixing with ambient air caused evaporation and shrinkage of particles, and accounts for differences between laboratories. Patients breathing from nebulizers may entrain ambient air possibly affecting deposition.