Medical aerosols may be delivered in combination with gases other than air, thus it is of interest to assess the effects of gas properties on the characteristics of the administered aerosol separately from effects on ventilation distribution and particle deposition mechanisms. This work investigated the influence of the supplied gas, either air or a mixture containing 78% helium and 22% oxygen, on droplet sizes produced by a vibrating mesh nebulizer incorporated in a combined gas-aerosol delivery system. Droplet size distributions were measured by laser diffraction. Nebulization was performed using three different meshes, producing droplets with nominal volume median diameters (VMDs) of 3, 4, or 5 mu m. Measured VMDs were stable, in that they were in all cases within +/- 10% of their nominal values, and unaffected by humidity or dilution of the aerosol stream. While VMDs were consistently 5-10% smaller in helium-oxygen than in air, this variation was small compared to the variation between meshes. Accordingly, unlike jet nebulizers, vibrating mesh nebulizers having high output rates can be operated in helium-oxygen with only minor impact on emitted droplet sizes. This will be attractive in the design of controlled clinical studies investigating aerosol delivery in helium-oxygen. In assessing such therapies, it is important to distinguish effects of gas properties on the characteristics of the administered aerosol from effects on particle and fluid mechanics influencing the regional distribution of aerosol in the lung. Use of an aerosol delivery device that is virtually unaffected by changing gas properties, such as that tested in the present study, is a straightforward way to make such a distinction. (C) 2010 Elsevier Ltd. All rights reserved.