Effect of multi-walled carbon nanotube (MWCNT) inhalation on pulmonary cytotoxicity at different time points after exposure. Lactate dehydrogenase (LDH) activity (U/L), an indication of cytotoxicity, was measured from the first fraction of collected bronchoalveolar lavage fluid. Data are expressed as percent (%) change from respective sham (dashed line – 100%). N.D. – not determined. #p<0.05 vs. other MWCNT depositions for a given time point only.