: Takenaka, S.; Dornhöfer-Takenaka, H.; Muhle, H.

Abstract
Introduction: Particulate material deposited within the alveolar region is cleared from the lung via the tracheobronchial tree, the blood and the lymph. It is generally assumed that clearance mechanisms are dependent on the chemical and physical properties of the inhaled particles as well as on the dosage and duration of exposure. Insoluble particles, such as fly ash and TiO2, are considered to be cleared from alveoli mainly via the tracheobronchial tree; however, it has also been proposed that the lymphatic system plays an important role. There is still some uncertainty as to the transloction pathways by which particles are moved from their alveolar deposition sites to the main clearance routes. The present long-term study was originally designed to detect eventual health hazards constituted by fly ash derived from th combustion of coal. TiO2, a reportedly 'inert' compound of low toxicity, served as the ference aerosol. The minor lung changes noted in rats exposed to fly ash were consistent with the observations made by Raabe et al. (1982). Moreover, we obtained interesting information concerning the alveolar distribution of different insoluble particle types