The use of Anderson cascade impaction to measure droplet sizes from a vented nebulizer connected directly to the impactor was examined by performing two sets of impactor measurements. In the first set, the impactor was operated in room temperature air. In the second set, the impactor was immersed in a cooled water bath at the same temperature as the aerosol exiting the nebulizer (10 degrees C). Normal saline was nebulized in five Pari LC Jet + nebulizers driven by a single Pulmo-Aide compressor, operating under ambient conditions of 35% RH and 22 degrees C. Impaction was done once steady temperatures were reached. When the impactor was operated in room temperature air, the air travelling through the impactor warmed from 10 degrees C at the entrance to the impactor to room temperature at the exit of the impactor. This heating resulted in significant droplet shrinkage due to humidification by evaporation from the droplets, since the impactor immersed in the cooled water bath gave an MMAD that was on average 70% larger than was obtained when the impactor was operated in room air (3.4 mu m vs 2.1 mu m). These results emphasize the need for caution when using impactors to measure nebulized hygroscopic aerosols, since even if these aerosols enter the impactor in vapor pressure equilibrium with their surrounding air, significant size changes can occur during transit through the impactor if the temperature of the aerosol differs significantly from that surrounding the impactor.