Occupational Exposure to Carbon Nanotubes and Nanofibers

particle retention predicted by the MPPD model(which includes the ICRP [1994] clearance model)[CIIT and RIVM 2006] compared to that of thefirst-order kinetic model used to estimate the factorof 10/1 [Snipes et al. 1989; Pauluhn 2010b].When this same method was applied to the ratLOAEL of 0.1 mg/m3 in the Ma-Hock et al. [2009]subchronic inhalation study, but using the particlesize data and rat minute ventilation specific forthat study (Table A–2 and Section A.2.2), similarhuman-equivalent estimates were obtained. Theslightly higher doses are due to the greater DF forthe MWCNT in the Ma-Hock et al. [2009] study. Inaddition, the POD from the Ma-Hock et al. [2009]study is based on a LOAEL (vs. NOAEL in Pauluhn[2010a], so an additional uncertainty factor wouldbe applied (as discussed in the next section). Ineach case, the estimates using an interspecies normalizingfactor based on the alveolar epithelial cellsurface area are lower by a factor of approximatelyfour. The estimates in Table A–13 are working lifetimehuman-equivalent concentrations to the ratNOAEL or LOAEL with no uncertainty factorsapplied to these values. Lower estimated rat lungdoses and human-equivalent lung doses and associatedworking lifetime concentrations would beexpected if using MPPD 2.1 and density