This study shows that particles are present on the broncho-alveolar epithelium predominantly phagocytized in macrophages which will be cleared eventually by particle transport mechanisms. The transport rate from the lower respiratory tract can be quantitated. But dissolution of particles in the lungs and the metabolic fate of the particles and their radioactive label in blood and in the GI tract have to be taken into account. The decreasing particle transport rate indicates a decreasing fraction of particles on the epithelium caused by an increasing number of particles penetrating the epithelial membrane during time.