Occupational Exposure to Carbon Nanotubes and Nanofibers

and oxidative degradation of the tubes occurred[Liu et al. 2010]. Kagan et al. [2010] reported thatin vitro myeloperoxidase, which is found in highconcentrations in polymorphonuclear neutrophils(PMN), degraded SWCNT. However, it is uncertainas to whether PMN-derived myeloperoxidasewould degrade SWCNT in vivo (e.g., in the lung)because of the following: (1) PMN recruitment afterSWCNT exposure is a transient rather than persistentresponse, (2) there is no strong evidence forSWCNT phagocytosis by PMN, and (3) SWCNTand MWCNT are found in the lungs of micemonths after pharyngeal aspiration [Shvedova etal. 2005; Mercer et al. 2008; Porter et al. 2010].Several animal studies have shown that the size(e.g., length) of MWCNT and SWCNT may have aneffect on their biological activity [Takagi et al. 2008;Poland et al. 2008; Muller et al. 2009]. Intraperitonealinjection of mice with long MWCNT (20 µm length),but not short MWCNT (< 5 µm length), caused granulomatouslesions on the diaphragm in a 2-weekpost-exposure study [Poland et al. 2008]. Fibroticperitoneal adhesions and mesothelioma were alsoobserved after exposure to MWCNT in which approximately28% of the tubes were > 5 µm in length[Takagi et al. 2008]. However, when rats were exposedto short MWCNT (< 1 µm length) by intraperitonealinjection, only acute inflammation wasobserved, with no evidence of mesothelioma overthe 2 year post-exposure period [Muller et al. 2009].Nagai et al. [2011] provided evidence that the carcinogenicpotential of MWCNT may be related tothe fiber-like properties and dimensions. Fischer344/Brown Norway (male and female, 6 wk old)were injected with doses of 1 or 10 mg of one offive types of MWCNT with different dimensionsand rigidity. The thin diameter MWCNT (~50nm) with high crystallinity caused inflammationand mesothelioma, whereas thick (~150 nm) ortangled structures (~2–20 nm) were less cytotoxic,inflammogenic, or carcinogenic. A specific mutationto tumor suppressor genes (Cdkn2a/2b) wasobserved in the mesotheliomas, which is similarto that observed in asbestos-associated mesotheliomasinduced by asbestos. In vitro studies withmesothelial cells showed that the thin MWCNTpierced cell membranes and caused cytotoxicity.Numerous studies have investigated the genotoxicproperties of CNT with results from in vitro assaysindicating that exposure to SWCNT and MWCNTcan induce DNA damage, micronuclei formation,disruption of the mitotic spindle, and induction ofpolyploidy [Li et al. 2005; Kisin et al. 2007; Muller etal. 2008a; Pacurari et al. 2008; Lindberg et al. 2009;Sargent et al. 2009; Asakura et al. 2010]. Other invitro studies of some MWCNT did not show evidenceof genotoxicity [Wirnitzer et al. 2009; Kim etal. 2011]. The presence of residual metal catalystswas also found to promote the generation of reactiveoxygen species (ROS), thereby enhancing thepotential for DNA damage [Pulskamp et al. 2007;Barillet et al. 2010]. The results from in vitro studieswith CNF have also shown that exposure can causegenotoxicity [Magrez et al. 2006; Lindberg et al.2009; Kisin et al. 2011] including aneugenic as wellas clastogenic events. In addition, low-dose, longtermexposure of bronchial epithelial cells to SW-CNT or MWCNT has been reported to transformthese cells to exhibit unregulated proliferation, lossof contact inhibition of division, enhanced migrationand invasion, and growth in solf agar [Stueckleet al. 20011]. When SWCNT-transformed epithelialcells were subcutaneously injected into thehind flanks of immunodeficient nude mice, smalltumors were observed at one week post-injection.Histological evaluation of tumors showed classiccancer cell morphology, including the presence ofmultinucleated cells, an indicator of mitotic dysfunction[Wang et al. 2011].When CNT and CNF are suspended in test media,agglomerates of various sizes frequently occur. Thisis particularly evident in test media used in recentstudies where animals have been exposed to CNTsuspensions by intratracheal instillation, intraperitonealinjection, or by pharyngeal aspiration (a techniquewhere particle deposition closely resemblesinhalation). The agglomerate size for CNT and CNFis normally smaller in a dry aerosol than when suspendedin physiological media. Evidence from toxicitystudies in laboratory animals indicates that14 NIOSH CIB 65 • Carbon Nanotubes and Nanofibers