Morphometric determination of the initial lung distribution of MWCNT 1 day post aspiration. Results show the distribution of MWCNT fiber burden in airways, alveolar and subpleural tissue regions of the lungs one day after aspiration. As shown by the different shades of bars, MWCNT fiber burden in the airways and alveolar regions were further subdivided into the airspaces of the region, tissue of the region and macrophages. Results are expressed as a percentage of the total lung burden. (Mean ± SE, N = 7).