To improve aerosol therapy regimens or to assess the health hazards of inhaled toxic particulates, the dose delivered to the human respiratory tract must be determined. The efficiency with which an aerosol is deposited in various compartments of the tract is a function of its mass median aerodynamic diameter, MMAD (Task Group on Lung Dynamics, 1966). If an airborne particle is hygroscopic, it will change in geometric diametrer and density as a result of water vapor uptake; its MMAD, therefore, will vary with location throughout the human respiratory tract. As a result, its dose distribution will differ from that for a nonhygroscopic (i.e., hydrophobic) aerosol of identical inhaled MMAD.