Particle deposition in human and canine tracheobronchial casts: A determinant of radon dose to the critical cells of the respiratory tract

The radiation dose to the sensitive cells of the bronchial epithelium from inhaled short-lived radon progeny depends critically on the efficiency with which the particles are deposited on the airway surfaces. Measurements of deposition for particles 50, 100, 180, and 400 nm in diameter have been carried out along three single pathways in full hollow airway casts of the human and canine tracheobronchial trees. The pathways are a major branch path, a minor branch path, and one which consists of alternating major and minor branches. The casts, prepared from whole lungs obtained at autopsy, extend to airways with diameters less than 1 mm. Monodisperse test aerosols were nebulized from either a Tc-99m-ferricoxide or Tc-99m-NaCl solution and size classified with an electrostatic classifier or formed by condensation of triphenyl phosphate onto Tc-99m-salt nuclei. Measured deposition fractions (eta(F)s) form a family of curves with eta(F) being highest for the 50 mm particles and lowest for 400 nm. This agrees with expectations for this particle size range where diffusion is the dominant deposition mechanism. The eta(F) pattern is strongly influenced by the morphometry and resulting airflow distribution. Results of the experiments obtained in the human cast are compared with two predictive deposition models. The model, that is more explicit with respect to now and geometric parameters, appears to be a marginally better predictor of the data.