Occupational Exposure to Carbon Nanotubes and Nanofibers

etween the estimated deposited and retained lungburdens. Thus, the mouse fibrotic lung responsewas observed at an administered lung dose thatwas similar to, or higher than, the rat lung dosesestimated at the LOAEL. This suggests a roughlysimilar dose-response relationship in the rat andmouse lungs to CNT, based on the limited data inthese two studies.As discussed above (Section A.7.3), the mouse lungresponses to CNF (at a 120 µg dose) included alveolarseptal thickening identified as pulmonaryfibrosis based on collagen deposition observed bySirius Red staining and the measured thicknessof the alveolar connective (septal) tissue [Murrayet al. 2012]. In the DeLorme et al. [2012] study,similar qualitative lung responses were observed atthe 25 mg/m 3 (as discussed in Section A.7.3). TheDeLorme et al. [2012] did not report fibrosis at 25mg/m 3 although the description of the responses isconsistent with early stage fibrosis reported in theMurray et al. [2012].NOAELs were reported for one type of CNF in De-Lorme et al. [2012] and for one type of MWCNT inPauluhn [2010a], which were 0.1 and 0.54 mg/m 3 ,respectively. It follows that the human-equivalentworking lifetime exposure estimates to the NO-AEL would be roughly 5-fold higher for the CNFthan that for the MWCNT (although not exactly,due to particle size differences and lung depositionestimates). Table A–13 shows estimates of humanequivalentconcentrations to effect levels in the Pauluhnand Ma-Hock subchronic inhalation studies,based on different assumptions in extrapolating therat lung dose to humans. The application of uncertaintyfactors (e.g., Table A–14) with the CNF usedin the DeLorme et al. [2012] study would result inestimated working lifetime no-effect levels in humansof roughly 1–4 µg/m 3 .142 NIOSH CIB 65 • Carbon Nanotubes and Nanofibers