To investigate lung clearance from the human tracheo-bronchial tree, W. Stahlhofen et al. performed experiments in which boluses of radioactively labelled aerosols were administered in small volumetric regions of the lungs. As not expected there was found a large amount of aerosol which was cleared slowly even when the bolus was injected at the very end of inhalation. To obtain more information about the site of deposition of this radioactive material, boluses of monodisperse nonlabelled particles were inspired to determine aerosol recovery after periods of breath-holding.