A method has been presented (Ferron et al., J. Aerosol Sci. 1991, 22, S863) to estimate the deposition of polydisperse hygroscopic aerosol particles in the human respiratory tract. This method uses data obtained from a hygroscopic particle lung deposition (HPLD) model (Ferron et al., J. Aerosol Sci. 1988, 19, 611). Here errors in this calculation method are estimated by comparing data obtained with the estimation method and data calculated with the HPLD model for CoCl2.6H20 particles. These differences are normally less than 10% for monodisperse and polydisperse aerosols. This estimation method is more easy to handle with respect to the HPLD model.