# Impact of carbon-based fibers morphologies on their carcinogenic potential

## TL;DR

The study investigates how the shape and size of carbon-based fibers affect their potential to cause cancer, finding that only certain types and sizes show weak carcinogenic effects.

## Contribution

The study examines the carcinogenic potential of various carbon-based fibers, including single- and multi-walled nanotubes and fragmented carbon fibers.

## Key findings

- Carbon fiber fragments and bundled MWCNTs showed weak carcinogenic potency.
- Non-bundled MWCNTs with diameters below 30 nm did not show clear carcinogenic effects.
- SWCNTs and MWCNTs with larger diameters showed very weak or weak carcinogenic potency.

## Abstract

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).

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.

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.

The online version contains supplementary material available at 10.1186/s12989-026-00663-y.