Long-term inhalation toxicity studies with multiwalled carbon nanotubes: closing the gaps or initiating the debate?

Carbon nanotubes (CNTs) are allotropes of carbon with a graphene cylindrical nanostructure and have novel physical properties that make them potentially useful for multiple commercial applications. In many ways, CNTs have recently become the material science ‘‘posterchild’’ for the emerging field of nanotechnology—based upon their seemingly unlimited potential applications, including in industrial and biomedical products. The CNT physical characteristics that are most frequently cited include extraordinary strength, unique electrical properties, efficient conductors of heat, and compatibility with biological systems—owing to their ease of functionality. It is important to note, however, that the future successful development of products with CNTs may be undermined by serious concerns regarding their potential toxicity. In this regard, the results of numerous pulmonary toxicity studies in animals or cell culture systems have indicated that exposures to single-walled carbon nanotubes (SWCNTs, single cylinder) (Lam et al., 2004; Shvedova et al., 2008; Warheit et al., 2004) or multiwalled carbon nanotubes (MWCNTs, multiple cylinders) (Muller et al., 2005) have produced significant lung injury and often at relatively low doses or concentrations. Virtually all the studies, however, have utilized nonphysiological routes of pulmonary exposures (intratracheal instillation or nasopharyngeal aspiration) or have implemented inhalation studies of short-term duration (Shvedova et al., 2008). Thus, the longer term, 90-day inhalation study reported by Ma-Hock et al. (2009) highlighted in this volume of Toxicological Sciences has special significance. The Ma-Hock paper represents the first long-term inhalation study in rodents with CNTs, either of the single-walled (SWCNT) or multiwalled (MWCNT) variety. In this study, Wistar rats were exposed to aerosols of MWCNT for 13 weeks at exposure concentrations of 0, 0.1, 0.5, or 2.5 mg/m. The physicochemical characteristics of the MWCNT test samples were reported to be the following: (1) MWCNT composition 1⁄4 90% carbon–10% metal oxide catalyst; (2) BET surface area 1⁄4 250–300 m/g; and (3) MWCNT dimensions—length range 1⁄4 0.1–10.0 lm and diameter range 1⁄4 5–15 nm. Following 13 weeks of aerosol exposures, the authors reported no (extrapulmonary) systemic organ toxicity, based upon histopathological evaluation criteria. This may be a particularly important finding as it relates to any potential effects of MWCNT particulates that may have transmigrated from alveolar regions to systemic vasculature following deposition in the distal lungs. Indeed, the absence of any pathological responses in major organs such as the liver, kidney, or heart following 90 days of exposure is noteworthy. However, as might be expected, significant impacts in the respiratory tract were evident and included hyperplastic responses in the nasal cavity and upper airways (larynx and trachea) concomitant with increased lung weights, multifocal granulomatous inflammation, histiocytic and neutrophilic inflammation, lipoproteinosis in alveoli, and lung-associated lymph nodes. It is interesting to note that the investigators did not report pulmonary fibrotic effects in exposed rats. At the lowest exposure level, namely 0.1 mg/m, there was minimal granulomatous-type inflammation in the lungs and lungassociated lymph nodes. As a consequence, a no-observed-effect concentration could not be established. These findings confirm the potency of MWCNT as a pneumotoxicant and provide the hazard criteria basis for establishing risk assessment determinations. Moreover, the study results, by a ‘‘physiological’’ route of exposure (i.e., inhalation), confirm in principle the findings of adverse pulmonary effects reported in earlier studies employing nonphysiological routes or short-term inhalation exposures. While the inhalation study by Ma-Hock et al. has set an important precedent for gauging the pulmonary effects of CNTs, future studies should be expanded to address additional relevant safety issues that were not fully investigated in this study.