Impact of carbon-based fibers morphologies on their carcinogenic potential

Our plain-English summary of Impact of carbon-based fibers morphologies on their carcinogenic potential, by A. Wagner, F. Schulz, A. Meyer-Plath, et al (Particle and Fibre Toxicology, 2026).

The study

• Authors: A. Wagner, F. Schulz, A. Meyer-Plath, F. Dahlmann, S. Rittinghausen, D. Schaudien
• Journal: Particle and Fibre Toxicology
• Published: 2026;23(1)
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Carbon based fibers are considered to exhibit a carcinogenic potency when inhaled into the deep lung. Mesotheliomas develop after intraperitoneal application of multi-walled carbon nanotubes (MWCNTs) exceeding a diameter of about 37 nm, whereas carcinogenic potency decreases for diameters below this threshold. While large MWCNT diameters are associated with a rigid fiber geometry, this study examined the effects of MWCNTs with smaller diameters ranging from 10 to 30 nm. Also, a sample of single-walled carbon nanotubes (SWCNTs) exhibiting single fiber diameters significantly below 10 nm and showing a flexible geometry was included since individual SWCNT fibers can aggregate to form bundles that exhibit increased rigidity. Additionally, the carcinogenic effect of pitch-based carbon fiber fragments was investigated. Carbon fibers are industrially produced with diameters larger than 4 µm and are thus not per se respirable. However, pitch-based fibers tend to break along their longitudinal axis, resulting in respirable fragments, partially of critical WHO dimensions. Four CNT samples with a geometric mean diameter (GMD) of 30 nm, 20 nm, 10 nm, and smaller than 10 nm, as well as one fragmented carbon fiber sample (GMD 1.3 µm) were intraperitoneally injected into rats in two dosages (0.1 × 109 and 1 × 109 WHO fibers or WHO-analog nanofibers) and observed for up to 24 months. A long amosite asbestos (GMD 0.37 µm) with known fiber-specific carcinogenic effect served as a positive control (0.1 × 109 WHO fibers).

Results: A small number of mesotheliomas occurred in all fiber types, but not at all dosages. For the carbon fiber material, a possible weak carcinogenic potency is seen at the higher dosage. For the SWCNT fiber, low number of mesotheliomas likewise suggest a weak carcinogenic potency. In the case of the MWCNT fiber with a GMD of 30 nm, very low number of mesotheliomas indicate a possible very weak carcinogenic potency. No clear carcinogenic potency was observed for the MWCNTs with GMDs of 20 nm and 10 nm.

Conclusions: Carbon fiber fragments and thin but bundled MWCNTs showed weak carcinogenic potency. Non-bundled MWCNTs with a diameter below 30 nm did not show clearcarcinogenic potency at a dose up to 1 × 109 WHO-analog nanofibers.

© 2026. The Author(s).

In plain English

This animal experiment tested whether carbon-based fibers injected into the abdominal cavity cause mesothelioma in rats. Male Wistar Han rats, 50 per group, received one of four carbon nanotube materials (one single-walled; three multi-walled, roughly 10, 20 and 30 nm wide) or pitch-based carbon fiber fragments. Each was given at about 100 million or 1 billion fibers, counted as WHO fibers for the carbon fiber and as WHO-analog nanofibers for the nanotubes, and the rats were observed for up to 24 months. Long amosite asbestos was the positive control. Mesotheliomas occurred in 17 of 50 amosite rats (34%), 5 of 50 high-dose carbon fiber rats (10%), at most 4 of 50 (8%) in any nanotube group, and none in the negative control group. Only the amosite count was marked statistically significant. The authors judged some tumors to be fiber-induced and others, including those in the roughly 10 and 20 nm groups, most likely spontaneous. They concluded that the carbon fiber fragments showed weak carcinogenic potential at the high dose. They also concluded that the diameter below which multi-walled nanotubes show no asbestos-like effect is likely between 20 and 30 nm.

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: a 24-month intraperitoneal (abdominal injection) carcinogenicity study in rats, with an additional 3-month interim study of inflammatory effects
• Who was studied: 600 male Wistar Han rats, 10 weeks old, in 12 groups of 50, followed for 24 months, plus 35 rats (5 per low-dose group and per control group) in a 3-month study. The study was run at the Fraunhofer Institute for Toxicology and Experimental Medicine (ITEM), Hannover, Germany. Study dates are not given.
• Exposure: Five carbon-based materials, each given at a low dose (0.1 × 10^9) and a high dose (1 × 10^9) of WHO fibers (carbon fiber) or WHO-analog nanofibers (nanotubes):
• a single-walled carbon nanotube (OCSiAl Tuball, partly bundled)
• multi-walled carbon nanotubes Nanocyl NC-7000 long (about 10 to 12 nm)
• multi-walled carbon nanotubes IFW CNT1-1 (about 18 to 20 nm)
• multi-walled carbon nanotubes USRN 20–30 (about 27 to 30 nm)
• ball-milled fragments of a mesophase pitch-based carbon fiber (Dialead K13D2U, mean diameter about 1.2 to 1.4 µm), given as two injections on consecutive days
  Controls: long amosite asbestos at 0.1 × 10^9 WHO fibers (positive control) and dispersion medium (negative control).
• Main results:
  • Amosite asbestos positive control: mesotheliomas in 17 of 50 rats (34%).
  • Carbon fiber fragments (Dialead K13D2U), high dose: mesotheliomas in 5 of 50 rats (10%). None occurred in the low-dose carbon fiber group.
  • Single-walled nanotube (OCSiAl Tuball): mesotheliomas in 1 of 50 rats (2%) at low dose and 4 of 50 rats (8%) at high dose.
  • Multi-walled nanotube USRN 20–30 (about 30 nm): mesotheliomas in 2 of 50 rats (4%) at low dose and 1 of 50 rats (2%) at high dose.
  • Multi-walled nanotube CNT1-1 (about 20 nm), low dose: mesotheliomas in 2 of 50 rats (4%). None occurred in the high-dose CNT1-1 group.
  • Multi-walled nanotube Nanocyl NC7000 long (about 11 nm): one mesothelioma at low dose, which the authors considered most likely spontaneous. No mesotheliomas occurred at high dose.
  • No mesotheliomas were seen in the negative (medium) control group, the low-dose carbon fiber group, the high-dose CNT1-1 group, or the high-dose Nanocyl NC7000 long group.
  • In the mesothelioma incidence table, only the amosite group's total (17) carried a statistical significance mark (p less than 0.001 compared with the control). The totals for the carbon fiber and nanotube groups carried no significance mark.
  • The authors interpreted mesotheliomas in the abdominal cavity as induced by the fibers: 2 of 50 rats in the high-dose carbon fiber group, and 1 of 50 rats in each of the USRN 20–30 low and high dose groups and the OCSiAl Tuball low and high dose groups.
  • Tumor subtypes differed by material. In the amosite group, 11 of 17 mesotheliomas were sarcomatoid and 5 of 17 were biphasic. In the other groups, most mesotheliomas were of the epithelioid subtype.
  • Granulomatous inflammation in the abdominal organs was statistically significantly more common than in the negative control (p less than 0.001) in all treated groups except the low-dose OCSiAl Tuball group.
  • No tumors were seen at 3 months, and the 3-month findings did not predict the 24-month outcome.
  • Other abdominal tumors were judged to occur independently of treatment. Their rates matched normal background rates in male Wistar rats of the same age.
• The authors' conclusion: The authors concluded:
• Fragments of the carbon fiber Dialead K13D2U showed weak carcinogenic potential at the high dose.
• The single-walled nanotube OCSiAl Tuball showed weak potential, and the multi-walled nanotube USRN 20–30 showed very weak potential, in the rat intraperitoneal test.
• Nanocyl NC7000 long and CNT1-1 showed no clear carcinogenic potential.
• The diameter below which multi-walled carbon nanotubes are not expected to have an asbestos-like effect is likely between 20 and 30 nm.
• Single-walled carbon nanotubes also bear a carcinogenic potential, but the mechanisms need further evaluation.
• Limitations the authors state:
  • The measured diameter of the single-walled nanotube sample was only approximate, because it was at the edge of the electron microscope's resolution.
  • For the single-walled nanotube sample, the number of single fibers may have been underestimated, so the dose may refer more to fiber bundles.
  • The dose calculation for the carbon fiber included some fibers 3 µm or thicker, so the actual nominal dose was 0.93 or 0.093 × 10^9 WHO fibers rather than the target.
  • The carbon fiber sample contained a large share of non-fiber particles. The authors said this was the best achievable material, because further milling would have lost WHO fibers.
  • The authors said further long-term studies with enough animals and a higher dose of 5 × 10^9 WHO-analog nanofibers are needed for tangled-type nanotubes. They also suggested testing a higher dose, or a long-term inhalation study, for the carbon fiber.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Carcinogenic, Mesothelioma, Amosite.

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