Thirteen-week study of toxicity of fiber-like multi-walled carbon nanotubes with whole-body inhalation exposure in rats

Our plain-English summary of Thirteen-week study of toxicity of fiber-like multi-walled carbon nanotubes with whole-body inhalation exposure in rats, by T. Kasai, Y. Umeda, M. Ohnishi, et al (Nanotoxicology, 2015).

The study

• Authors: T. Kasai, Y. Umeda, M. Ohnishi, H. Kondo, T. Takeuchi, S. Aiso, T. Nishizawa, M. Matsumoto, S. Fukushima
• Journal: Nanotoxicology
• Published: 2015;9(4):413-22
• Cited by: 75 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Cancer development due to fiber-like straight type of multi-walled carbon nanotubes (MWCNTs) has raised concerns for human safety because of its shape similar to asbestos. To set concentrations of MWCNT for a rat carcinogenicity study, we conducted a 13-week whole body inhalation study. F344 male and female rats, 6-week-old at the commencement of the study, were exposed by whole-body inhalation to MWCNT at concentrations of 0, 0.2, 1 and 5 mg/m(3) with a generation and exposure system utilizing the cyclone sieve method. Measured concentrations in the exposure chambers were 0.20 ± 0.02, 1.01 ± 0.11 and 5.02 ± 0.25 mg/m(3) for 13 weeks. The MMAD (GSD) of MWCNT were 1.4-1.6 μm (2.3-3.0), and mean width and length were 94.1-98.0 nm and 5.53-6.19 μm, respectively, for each target concentration. Lung weights were increased 1.2-fold with 1 mg/m(3) and 1.3-fold with 5 mg/m(3) in both sexes compared to the controls. In the bronchoalveolar lavage fluid (BALF) analyses, inflammatory parameters were increased concentration-dependently in both sexes from 0.2 mg/m(3). Granulomatous changes in the lung were induced at 1 and 5 mg/m(3) in females and even at 0.2 mg/m(3) in males. Focal fibrosis of the alveolar wall was observed in both sexes at 1 mg/m(3) or higher. Inflammatory infiltration in the visceral pleural and subpleural areas was induced only at 5 mg/m(3). In conclusion, we determined 0.2 mg/m(3) as the low-observed-adverse-effect level (LOAEL) for respiratory tract toxicity in the present inhalation exposure study of rats.

In plain English

This was an animal experiment on carbon nanotubes, not on asbestos or people. To choose doses for a later 2-year cancer study, the authors exposed groups of 10 male and 10 female F344 rats to airborne fiber-like straight multi-walled carbon nanotubes (MWCNT), whose shape resembles asbestos. The rats breathed 0, 0.2, 1 or 5 mg/m3, 6 hours a day, 5 days a week, for 13 weeks. Lung weights rose at 1 and 5 mg/m3. Lung-wash fluid showed concentration-related increases in inflammation markers in exposed rats. Granulomatous changes were found in 8 of 10 males and 4 of 10 females at 1 mg/m3, in all rats at 5 mg/m3, and in one male at 0.2 mg/m3. Focal fibrosis of the alveolar wall appeared at 1 mg/m3 and above. At 5 mg/m3, one rat of each sex had inflammation of the visceral pleura. Nanotubes reached the pleura at the diaphragm, but the mesothelium looked unchanged. The authors set 0.2 mg/m3 as the lowest level with an observed adverse effect. They said their ongoing 2-year cancer study should clarify long-term effects.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal experiment: 13-week whole-body inhalation toxicity study in rats, run as a preliminary dose-range-setting study for a 2-year carcinogenicity study
• Who was studied: Male and female F344/DuCrlCrlj rats from Charles River Japan, 6 weeks old at the start. There were 10 rats per sex in each of three exposed groups and in a clean-air control group, for 80 rats in total. The work was done at the Japan Bioassay Research Center, Japan. Study dates are not given.
• Exposure: Fiber-like straight multi-walled carbon nanotubes (Hodogaya Chemical MWNT-7). They were given as a dry aerosol at target concentrations of 0.2, 1 and 5 mg/m3, 6 h/day, 5 days/week, for 13 weeks (62 exposures). Measured concentrations were 0.20, 1.01 and 5.02 mg/m3. Airborne fibers had mean widths of 94.1 to 98.0 nm. Mean lengths are printed as 5.53 to 6.19 'mm' in the scanned text; the PubMed abstract gives micrometres. No asbestos was tested.
• Main results:
  • No deaths or abnormal clinical signs occurred in any exposed or control rats during the 13 weeks.
  • Lung weights were significantly increased in males and females at 1 and 5 mg/m3, at 1.2-fold and 1.3-fold of control values. Male left lung weights were 0.43 g (control), 0.45, 0.51 and 0.56 g. Female left lung weights were 0.32 g (control), 0.33, 0.37 and 0.42 g.
  • In lung-wash (BALF) fluid, neutrophils and lymphocytes increased with concentration in all exposed rats of both sexes. LDH, ALP, total protein and albumin also showed concentration-related significant increases in all exposed groups, except ALP in females at 0.2 mg/m3.
  • Granulomatous change in the lung was seen in males in 0, 1, 8 and 10 of 10 rats at 0, 0.2, 1 and 5 mg/m3. In females it was seen in 0, 0, 4 and 10 of 10 rats. Significance markers appear for males at 1 and 5 mg/m3 and for females at 5 mg/m3 only.
  • At 0.2 mg/m3, the only granulomatous finding was one small lesion in one male rat, with no focal fibrosis around it. The authors nonetheless considered it an adverse effect of exposure.
  • Focal fibrosis of the alveolar wall was seen in males in 0, 0, 10 and 10 of 10 rats at 0, 0.2, 1 and 5 mg/m3. In females it was seen in 0, 0, 9 and 10 of 10 rats.
  • Inflammatory infiltration in the visceral pleura and subpleural areas occurred in one rat of each sex, only in the 5 mg/m3 group.
  • Nanotubes were seen by polarizing microscopy in the visceral subpleural areas and in the parietal pleura at the diaphragm. No change in the mesothelium was seen by light microscopy.
  • Goblet cell hyperplasia of the nasal cavity occurred in all 10 rats of each sex at 1 and 5 mg/m3.
  • Mean nanotube amounts in the left lung were 3.23, 21.2 and 120.3 micrograms in males at 0.2, 1 and 5 mg/m3, and 2.30, 13.7 and 80.3 micrograms in females. The scan prints 'mg', but the article's whole-lung estimate of 0.00969 mg at 0.2 mg/m3 confirms micrograms. Male amounts were 1.4 to 1.6 times female amounts.
• The authors' conclusion: The authors concluded that the fibrous MWCNT caused concentration-dependent subchronic effects, such as granulomatous change, focal fibrosis and changes in BALF parameters. They set the lowest-observed-adverse-effect level (LOAEL) at 0.2 mg/m3, based on granulomatous changes and BALF parameters. They also calculated a benchmark concentration lower limit (BMCL) of 0.056 mg/m3.
• Limitations the authors state:
  • Polarizing microscopy was done only on the lung and diaphragm. Because nanotubes were seen in diaphragm muscle capillaries, the authors noted that nanotubes may have spread elsewhere in the body.
  • The 13-week study cannot show long-term or cancer effects. The authors pointed to their ongoing 2-year carcinogenicity study for this.

Topics

Medical terms in this study, explained in our Lexicon: Bronchoalveolar Lavage (BAL), Fibrosis.

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