Morphometric determination of the lung distribution of MWCNTs 56 days post- aspiration. Results show the distribution of MWCNT burden in airways, alveolar and subpleural tissue regions of the lungs 56 days after aspiration of an 80 ug dose. Results are expressed as a percentage of the total lung burden. As shown in the graph, MWCNTs within, partially within or completely penetrating alveolar macrophages account for the majority of the MWCNT lung burden. Data are means ± SE for an N of 8 animals per group.