The amount of aerosol inhaled in different fine particle definitions is compared to the amount of aerosol depositing in the lung and alveolar regions for nearly isotonic nebulized aerosols. These comparisons are made using a two-way coupled hygroscopic deposition model with experimental data obtained for over 200 different nebulizers from 19 different nebulizer models nebulizing salbumatol sulphate (1 mg ml(-1) in isotonic saline). Mass depositing in the lungs and mass inhaled in particles with diameters 1-6 mu m are statistically different (p < 0.05). Results are also presented for log-normally distributed aerosols as a function of mass median aerodynamic diameter (MMAD) ranging from 1.0 to 9.0 mu m and geometric standard deviations (GSDs) from 1.0-2.5. Fine particle fraction definitions of 0-5, 1-5, 0-6, 1-7 and 1-10 mu m are considered. For the polydisperse aerosols considered, each inhaled fine particle mass is equal to mass depositing in the lungs at a unique critical MMAD. However, all of the inhaled fine particle definitions give results much different from mass depositing in the lungs at MMADs away from their critical MMADs, overestimating mass depositing in the lungs by as much as 3.7 times and underestimating it by as much as 6.3 times. Although mass depositing in the lungs and the different fine particle definitions are correlated over certain particle size ranges, no correlation is valid over the entire particle size range considered. For reasonably polydisperse aerosols, inhaled fine particle mass strongly overemphasizes the benefits of MMADs in the 2-4 mu m range, since lung deposition is much less weakly dependent on MMAD than predicted by fine particle dose.