Respiratory toxicity of multi-wall carbon nanotubes

Our plain-English summary of Respiratory toxicity of multi-wall carbon nanotubes, by J. Muller, F. Huaux, N. Moreau, et al (Toxicology and Applied Pharmacology, 2005).

The study

• Authors: J. Muller, F. Huaux, N. Moreau, P. Misson, J.F. Heilier, M. Delos, M. Arras, A. Fonseca, J.B. Nagy, D. Lison
• Journal: Toxicology and Applied Pharmacology
• Published: 2005;207(3):221-31
• Cited by: 620 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Carbon nanotubes focus the attention of many scientists because of their huge potential of industrial applications, but there is a paucity of information on the toxicological properties of this material. The aim of this experimental study was to characterize the biological reactivity of purified multi-wall carbon nanotubes in the rat lung and in vitro. Multi-wall carbon nanotubes (CNT) or ground CNT were administered intratracheally (0.5, 2 or 5 mg) to Sprague-Dawley rats and we estimated lung persistence, inflammation and fibrosis biochemically and histologically. CNT and ground CNT were still present in the lung after 60 days (80% and 40% of the lowest dose) and both induced inflammatory and fibrotic reactions. At 2 months, pulmonary lesions induced by CNT were characterized by the formation of collagen-rich granulomas protruding in the bronchial lumen, in association with alveolitis in the surrounding tissues. These lesions were caused by the accumulation of large CNT agglomerates in the airways. Ground CNT were better dispersed in the lung parenchyma and also induced inflammatory and fibrotic responses. Both CNT and ground CNT stimulated the production of TNF-alpha in the lung of treated animals. In vitro, ground CNT induced the overproduction of TNF-alpha by macrophages. These results suggest that carbon nanotubes are potentially toxic to humans and that strict industrial hygiene measures should to be taken to limit exposure during their manipulation.

In plain English

This was an animal and laboratory experiment on multi-wall carbon nanotubes, not on asbestos itself. Chrysotile asbestos and carbon black were used as comparison materials. Female Sprague–Dawley rats had intact or ground nanotubes (0.5, 2 or 5 mg) placed directly into the windpipe. Comparison rats received 2 mg of chrysotile, 2 mg of carbon black, or saline. After 60 days, 81.2% of the intact nanotubes and 36.0% of the ground nanotubes (0.5 mg dose) were still in the lungs, compared with the amount measured on the day they were given. Both forms caused lung inflammation and raised collagen, a sign of scarring (fibrosis). Intact nanotubes formed collagen-rich lumps (granulomas) that partly or fully blocked airways. Ground nanotubes spread into lung tissue. Chrysotile also caused fibrosis; carbon black did not. In cell cultures, ground nanotubes, chrysotile and carbon black raised the inflammatory signal TNF-alpha. Intact nanotubes did not, and they floated away from the cells. The authors concluded that nanotubes reaching the lung persist and can cause inflammation and fibrosis. They called for further studies, including inhalation studies.

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 in rats (intratracheal instillation) plus in vitro tests on rat peritoneal macrophages
• Who was studied: Female Sprague–Dawley rats weighing 200–250 g (5 rats per group for the lung-persistence measurements; 4–6 observations per group for inflammation and fibrosis measures); peritoneal macrophages from rats for the cell-culture tests (3 observations per group)
• Exposure: Purified multi-wall carbon nanotubes, intact (average length 5.9 micrometres) or ground in a ball mill (average length 0.7 micrometres), at 0.5, 2 or 5 mg per rat. Comparison groups received 2 mg of UICC Rhodesian chrysotile A asbestos, 2 mg of ultrafine carbon black, or saline. In cell culture, the doses were 20, 50 and 100 micrograms.
• Main results:
  • Lung persistence (0.5 mg dose): 81.2% of intact nanotubes and 36.0% of ground nanotubes were still in the lungs at day 60, relative to day 0.
  • Inflammation at day 3: intact and ground nanotubes caused a dose-dependent rise in LDH (a marker of cell damage) in lung washings, more marked with ground nanotubes, and an accumulation of neutrophils and eosinophils. Chrysotile raised LDH; carbon black did not.
  • Fibrosis at day 60: hydroxyproline rose significantly and with dose after intact nanotubes. For ground nanotubes, only the 5 mg dose raised hydroxyproline significantly.
  • Type I collagen in the lung rose significantly after chrysotile, intact nanotubes and ground nanotubes compared with control rats, but not after carbon black.
  • Microscopy at day 60: intact nanotubes produced collagen-rich granulomas in the bronchi that partly or completely blocked the airway.
  • Microscopy at day 60: ground nanotubes were better spread through the lung and produced granulomas in the interstitial tissue.
  • Microscopy at day 60: chrysotile caused thickening of lung tissue with fibrosis and alveolitis. Carbon black only accumulated in macrophages, without significant change to lung structure.
  • TNF-alpha in lung washings rose at day 3 after chrysotile, carbon black, 2 mg intact nanotubes and ground nanotubes. At day 60 the rise persisted only in the chrysotile and ground-nanotube (0.5 and 2 mg) groups.
  • In cell culture, chrysotile, carbon black and ground nanotubes, but not intact nanotubes, caused dose-dependent cell damage and increased TNF-alpha. The intact nanotubes formed floating clumps that did not touch the cells.
• The authors' conclusion: The authors concluded that if multi-wall carbon nanotubes reach the lung, they persist, stimulate lung cells to produce TNF-alpha, and cause lung inflammation and fibrosis. They added that the precautionary principle should be applied to limit human exposure during handling.
• Limitations the authors state:
  • Placing a single bolus of tangled material into the windpipe may produce artificial granulomas in the airways.
  • The persistence data for intact nanotubes may be biased by artificial retention of large clumps of material.
  • The lack of effect of intact nanotubes in cell culture was most probably due to the nanotubes not reaching the cells.
  • Further studies are needed in other animal models and by other routes, such as inhalation, to account for whether the material can be breathed in.

Topics

Medical terms in this study, explained in our Lexicon: Inflammation, Fibrosis, Collagen, Granuloma, Parenchyma, Macrophage, Industrial Hygiene.

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