Multiwalled carbon nanotubes (MWCNTs) are highly ordered hexagonal lattices of carbon atoms arranged into cylinders by hydrogen bonding, dipolar forces, hydrophilic or hydrophobic interactions, gravity, and other forces. MWCNTs are synthesized by applying energy to a carbon source, which produces individual or groups of carbon atoms that reassemble into tubes. One of the primary uses of MWCNTs is in nanotube-reinforced polymer composite materials that take advantage of their low-density and high load-bearing capacity. Nanoscale materials were nominated by the Rice University Center for Biological and Environmental Nanotechnology to the National Toxicology Program for toxicologic testing. Because long-term inhalation toxicity and carcinogenicity studies were being conducted on a relatively short, rigid MWCNT, a representative long and thin MWCNT was selected for these studies. Following an evaluation of 24 different long, thin MWCNTs, the 1020 Long Multiwalled Carbon Nanotube (L-MWNT-1020) (Sun Innovations, Fremont, CA) was selected for study based on availability, high purity (97%), and the low amount of residual nickel catalyst (0.52% by weight). The average L-MWNT-1020 nanotube length was 2,600 nm and the average width was 15.3 nm. Because nickel was shown to be tightly bound to L-MWNT-1020, tissue nickel content was measured to determine lung burden.
The objective of these 30-day studies was to obtain lung burden, clearance, and toxicity data to be used in setting exposure concentrations for a chronic inhalation study of L-MWNT-1020. For lung burden studies, groups of 25 male and 25 female rats and mice were exposed to L-MWNT-1020 by whole-body inhalation at concentrations of 0, 0.1, 0.3, 1, 3, or 10 mg/m3, 6 hours plus T90 (12 minutes) per day, 5 days per week for 30 (males) or 31 (females) days, and necropsied on the last exposure day or after 14, 42, or 126 days of recovery. For the core studies, groups of 10 male and 10 female rats and mice were exposed to L-MWNT-1020 at the same concentrations as the lung burden studies for the same length of time and necropsied on the last exposure day.
L-MWNT-1020 lung burdens in rats and mice generally decreased with increasing postexposure time but were quite variable across the exposed groups. L-MWNT-1020 clearance rates were similar in 0.1 and 0.3 mg/m3 male and female rats with half-lives in the range of 52 to 64 days in male and female rats and 47 to 96 days in male and female mice. Clearance from the lung became progressively slower at exposure concentrations of 1 mg/m3 (rats) or 3 mg/m3 (mice) and above, with half-lives increasing to 465 to 583 days in rats exposed to 10 mg/m3 of L-MWNT-1020 and to 337 to 649 days in mice exposed to 10 mg/m3. Conditions for lung overload were achieved in the 3 and 10 mg/m3 dose groups, which is consistent with the extended clearance half-lives observed in those groups.
All core study rats and mice survived until the end of these studies, and there were no statistically significant differences in body weights in any exposed group relative to the respective chamber controls. There were no clinical findings related to exposure. Brown discoloration of the lungs was observed in most rats exposed to 3 mg/m3 of L-MWNT-1020 and in all rats exposed to 10 mg/m3 and male and female mice exposed to 3 or 10 mg/m3. In addition, the bronchial and mediastinal lymph nodes were enlarged and discolored (brown or gray) in all rats exposed to 10 mg/m3, in most rats exposed to 3 mg/m3, and in some rats exposed to 1 mg/m3. Brown discoloration of bronchial and mediastinal lymph nodes was observed in most exposed female mice and in all male mice exposed to 3 or 10 mg/m3. Lung weights of female rats exposed to 10 mg/m3 and male and female mice exposed to 3 and 10 mg/m3 were significantly greater than those of the respective chamber controls.
In the nose of mice, the incidences of accumulation of hyaline droplets in the olfactory epithelium were significantly increased in females exposed to 3 mg/m3 of L-MWNT-1020 and in males and females exposed to 10 mg/m3, and the incidences of this lesion in the respiratory epithelium were significantly increased in females exposed to 1 mg/m3 and in males and females exposed to 3 and 10 mg/m3. Nasal cavity lesions were not present in exposed rats. In the larynx, the incidences of laryngeal epiglottic squamous metaplasia were significantly increased in female rats and male mice exposed to 1 mg/m3, and in male and female rats and mice exposed to 3 and 10 mg/m3 compared to those in the respective chamber controls. In the lungs of rats and mice, the incidences of chronic inflammation were significantly increased in males and females exposed to 1, 3, and 10 mg/m3. Alveolar epithelial hyperplasia was observed in three male and two female rats exposed to 1 mg/m3 and in all male and female rats in the 3 and 10 mg/m3 exposure groups. Bronchiolar epithelial hyperplasia occurred in only male and female mice exposed to 3 and 10 mg/m3, and the incidences were significantly increased compared to those in the chamber controls. The incidences of alveolar proteinosis in females exposed to 3 mg/m3 and in male and female mice exposed to 10 mg/m3 were significantly increased compared to those in the chamber controls; this lesion was not present in exposed rats.
Lymphoid hyperplasia was observed in the bronchial and mediastinal lymph nodes of exposed male and female rats and mice. The incidences of lymphoid hyperplasia in both lymph nodes were significantly increased in the male rats exposed to 3 and 10 mg/m3 of L-MWNT-1020 (except bronchial lymph node in the 10 mg/m3 dose group) and in female rats, as was the incidence in the mediastinal lymph node of female rats exposed to 1 mg/m3. In mice, lymphoid hyperplasia of the bronchial and mediastinal lymph nodes occurred in males exposed to 1, 3, and 10 mg/m3 but not in chamber controls. The incidence of bronchial lymphoid hyperplasia in males exposed to 3 mg/m3 was significantly increased compared to that in the chamber controls. In female mice, lymphoid hyperplasia of the bronchial and mediastinal lymph nodes occurred in the 1, 3, and 10 mg/m3 dose groups; one control female had lymphoid hyperplasia in the bronchial lymph node. In females exposed to 10 mg/m3, the incidences in both lymph nodes were significantly increased compared to those in the chamber controls.
The most severe respiratory tract lesions occurred in rats and mice exposed to concentrations that caused lung overload (3 mg/m3 and above). Exposure to concentrations below 3 mg/m3 caused many of these same lesions, although at lower incidences and severity. Relative to chamber controls, there were no significantly increased incidences of lesions in the respiratory tract of animals exposed to 0.3 mg/m3 or lower. Consistent with these results, 0.3 mg/m3 is considered the no‑observed‑adverse‑effect level for L-MWNT-1020.
National Toxicology Program (NTP). 2019. NTP technical report on the toxicity studies of 1020 long multiwalled carbon nanotubes (L-MWNT-1020) administered by inhalation to Sprague Dawley (Hsd:Sprague Dawley SD) rats and B6C3F1/N mice. Research Triangle Park, NC: National Toxicology Program. Toxicity Report 94. https://doi.org/10.22427/NTP-TOX-94.