Proposition of clearance mechanism of MWCNT in the lung. MWCNT could be engulfed by macrophages (a), hydroxylyzed and cleaved (b). After this step, some cells enter into apoptotic phase (c). Then, a new cell could engulf the apoptotic macrophages to continue the elimination (d). The last step will be repeated as necessary to eliminate totally the MWCNT from the lung (e).