Purpose. To examine the effect of particle size and morphology on aerosol dispersion using jet-milled and spray-dried mannitol particles in narrow size distributions within the respirable range. Methods. Particle size and morphology were examined by laser diffraction and scanning electron microscopy, respectively. Aerosol dispersion was examined using a cascade impactor with a preseparator operating at a flow rate of 60 L/min, using two inhaler devices: Rotahaler (low-resistance device) and Inhalator ( high- resistance device). Powder flow was examined using static and dynamic methods (Carr's compressibility index and vibrating spatula, respectively). Results. Narrow size distributions of jet-milled and spray-dried particles were produced (d(50%) = 1.4 to 10.3 mum, GSD = 1.8 to 2.1, and d(50%) = 1.6 to 7.5 mum; GSD = 1.5 to 1.9, respectively). All particles were highly crystalline. Differences in particle shape were observed between jet-milled and spray-dried particles. Higher fine particle fraction (FPF) and relative fine particle fraction (FPFrel) ( greater aerosol dispersion) and lower geometric standard deviation ( GSD) ( less variation) were obtained using particles with d50% between 2 and 5 mum. Higher mass median aerodynamic diameter were obtained with larger d(50%). Spray-dried particles produced greater aerosol dispersion compared with jet-milled particles. Greater aerosol dispersion was obtained using the Inhalator than the Rotahaler. Conclusions. Small changes in the particle size within the 1 - 10-mum range produced a major impact in the aerosol dispersion of jet-milled and spray-dried particles. Even in these narrow size ranges, aggregation plays an important role in aerosol dispersion
