Time course of MWCNT penetrations into the subpleural tissue and intrapleural space. There was a significant number of penetrations into the subpleural tissue and intrapleural space with an initial spike at one day after aspiration with some initial clearance evident by the decline at 7 days. However the number of penetrations in both spaces significantly increased after day 7 and was still elevated 56 days after the initial aspiration. Data from animals given a single aspiration dose of 80 μg. As indicated by the asterisk, the number of subpleural tissue penetrations at 1 day was significantly different from the number of penetrations at 7, 28 and 56 days. (Mean ± SE, N = 8).