Multi-Walled Carbon Nanotubes (MWCNT), synthetic graphite in tubular shape

Endpoint summary

Additional information

All relevant and available information on intrinsic properties of MWCNT are cited in chapter 7 of the IUCLID. Although a large number of studies on the toxicity of MWCNTs have recently been published, clear characterization of the test materials including sample preparation which are quite essential for ensuring reproducibility and reliability in the toxicity test using suspension, in vivo and in vitro methods, is often missing. Importantly, modifications of some specific characteristics of the MWCNT tested such as variations in particle size distribution or length might give rise to differences in the toxicological profile. 

Toxic effects of different MWCNTs seem to depend on the form (length) and physico-chemical properties (metal content, aggregation/agglomeration, surface chemistry, and functionalisation). Thus for the time being, a case-by-case approach would be appropriate and the hazard assessment for MWCNT meeting the form described in section 4.5 of the IUCLID dossier is based solely on the information collected with this MWCNT. That is the reason why a comprehensive documentation in the IUCLID dossier and the chemical safety report involved automatically is restricted to all available and relevant data on the MWCNT meeting the form described in section 4.5 of the IUCLID dossier. That means on the other hand that the values used for the risk assessment exercise should not be used in general for all MWCNTs or CNTs, as the results obtained with one particular type of MWCNTs may not necessarily be relevant for other CNTs with other dimensions and properties

In addition the information on MWCNT not meeting the form described in section 4.5 of the IUCLID dossier (the differences can be tracked back to the original papers or because in others the characterization of the materials was unclear or incomplete) is also included in the IUCLID dossier but the entries are restricted to the information on data source to give a quick overview of all available data.

 

In a subchronic toxicity study (OECD TG 413) 10 male and 10 female Wistar rats per dose group were nose-only exposed for 13 weeks (6 hours /day, 5 days/week) to a respirable solid aerosol of test substance meeting the form described in section 4.5 of the IUCLID dossier (Pauluhn, 2010a, b).

Micronization of MWCNT was applied to increase the dustiness of the test material without destroying the assemblage structure of MWCNT. This was verified by characterization the MWCNT before and after micronization.The aerosolized MWCNT was also confirmed to share essentially identical composition and morphology with the non-dispersed/non-micronized MWCNT.

The animals were exposed to 0, 0.1, 0.4, 1.5 and 6 mg/m³. Additional 30 male rats were exposed in the air control and all exposure groups followed by a maximum recovery period of 6 months. An interim sacrifice group of 6 animals per air control and dose group was necropsied after 8 weeks. Recovery groups wre sacrificed 4, 13 and 26 weeks after the end of the exposure period, respectively.

All exposures were tolerated without effect. Indeed conclusive changes in body weights, food and water consumption, or specific clinical findings did not occur. Reflexes and body temperatures were indistinguishable between the groups. A concentration-dependent and sustained increase of absolute end relative lung weights occurred at 0.4 mg/m³ and above. The increased weights of the lung associated-lymph-nodes (LALN) mirrored the respective changes in lung weights which is consistent with a concentration-dependent increase of the particle clearance via lymphatic pathways. Equivocal changes LALN weights were already noticed at 0.1 mg/m³. With regard to bronchoalveolar lavage (BAL ) endpoints, 0.1m/m³ is considered to be a no-observed adverse effect level (NOAEL). There was only mild attenuation in the severity of the inflammatory response probed by BAL at 0.4 mg/m³. However, all end points suggestive of pulmonary inflammation were statistically indistinguishable from the control at the end of the 6-moths postexposure period. Histopathology demonstrated a complementary picture to BAL-analysis. After the 13 -week exposure period, goblet cell hyper- and/or metaplasia, eosinophilic globules and focal turbinate remodeling (thickening of turbinate bone with increased activity of osteoblasts) occurred in the nasal cavities at 1.6 mg/m³ and above. Minimal eosinophilic globules or minimal goblet cell hyper-/metaplasia ware apparent with low incidence in some rats at 0.4 mg/m³. In the bulbus oIfactorius neither evidence ofparticle translocation nor reactive findings existed. In the lungs, black particle-laden alveolar macrophages were observed. Macrophages had an enlarged and/or foamy appearance at hlgher exposure levels. Black particles were observed both in the alveoli as well in the interstitium. Hypercellularily of airway epithelial cells at the bronchiolo-alveolar junction, including an increased influx of inflammatory cells and septal thickening, occurred in a concentration-dependent manner at 0.4 mg/m³ and above. Slight to moderate inflammation, focally with granulomatous appearance, occurred in addition at 6 mg/m³. Sirius-red stained and Masson-Trichrome lungs showed increased interstitial collagen at 1.5 and 6.0 mg/m³. Focally increased collagen was also detected adjacent to areas of increased substance deposition and inflammatory infiltrates at 0.4 mg/m³. In LALNs black pigmentation occurred at 0.4 mg/m³ and above. LALNs showed increased cellularity of the paracortex. Thickening of the visceraI pleura, due to inflammation, collagen deposition, and increased black particle load, was apparent in rats exposed at 1.5 mg/m³and above. Black macrophages in the BAL T and increased BALT were observed at all particle-exposure levels. During the course of the 6 months recovery period, black pigment in alveolar macrophages was detectable in all substance exposed rats. Likewise, black macrophages were present in the BALT. Inflammatory findings of the lungs did not decreased in severity to any significant extent with increasing postexposure duration. A reactive, multifocal bronchiolo-alveolar hyperplasia was slightly increasing during the recovery period at 6 mg/m³; however, without consistent time-related trend. Apart from LALNs at no exposure concentration extrapulmonary toxicity was observed.Collectively, it is concluded that 0.1 mg/m³ constitutes the NO(A) EL in this study based on nasal and pulmonary inflammatory responses (this means at sites of predominant particle deposition).

 

A second subchronic inhalation study confirms the absence of any pathological response in major organs such as the liver, kidney or heart and confirms in principle the findings of adverse pulmonary effects (Ma-Hock, 2008).The study reported by Ma-Hock et al. however did not contain a postexposure period which is required to assess sustained pulmonary inflammation and recovery and to get as much information as possible to better appreciate the primary mode of action of the particle examined. In addition pulmonary toxicity was not addressed by bronchoalveolar lavage (BAL) although the counts of polymorphonuclear cells (PMNs) is considered to be the most sensitive biochemical endpoint in inhalation studies with poorly soluble particles. The fact that these important parameters/evaluations are not addressed in the Ma-Hock study are the reason why the risk assessment will be based on the study reported by Pauluhn (2010a, b).

Justification for classification or non-classification

No classification required according to EU-Directive 67/548/EEC, Annex VI.

No classification required according to Regulation (EC) No 1272/2008, Annex I.