[en] A slow bronchial clearance phase, describing the removal of insoluble particles from the sol layer has recently been incorporated into the ICRP human respiratory tract model. A striking feature of that clearance phase is the size dependence of the slowly cleared fraction. In the theoretical approach presented here, it is suggested that the slow bronchial clearance phase includes three main mechanisms: (1) re-transfer of particles onto the gel layer after a certain time delay, (2) intra- or intercellular transfer of particles through the airway epithelium and subsequent transport to the blood or lymph nodes, and (3) uptake of particles by airway macrophages. The experimentally observed inverse relationship between the slow clearance fraction and geometric particle size is interpreted as an increasing capture probability in the sol layer and transepithelial transport with decreasing particle diameter. (author)