Occupational Exposure to Carbon Nanotubes and Carbon Nanofibres: More Than a Cobweb

Our plain-English summary of Occupational Exposure to Carbon Nanotubes and Carbon Nanofibres: More Than a Cobweb, by E. Bergamaschi, G. Garzaro, G. Wilson Jones, et al (Nanomaterials, 2021).

The study

• Authors: E. Bergamaschi, G. Garzaro, G. Wilson Jones, M. Buglisi, M. Caniglia, A. Godono, D. Bosio, I. Fenoglio, I. Guseva Canu
• Journal: Nanomaterials
• Published: 2021;11(3)
• Type of article: Review
• Cited by: 37 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are erroneously considered as singular material entities. Instead, they should be regarded as a heterogeneous class of materials bearing different properties eliciting particular biological outcomes both in vitro and in vivo. Given the pace at which the industrial production of CNTs/CNFs is increasing, it is becoming of utmost importance to acquire comprehensive knowledge regarding their biological activity and their hazardous effects in humans. Animal studies carried out by inhalation showed that some CNTs/CNFs species can cause deleterious effects such as inflammation and lung tissue remodeling. Their physico-chemical properties, biological behavior and biopersistence make them similar to asbestos fibers. Human studies suggest some mild effects in workers handling CNTs/CNFs. However, owing to their cross-sectional design, researchers have been as yet unable to firmly demonstrate a causal relationship between such an exposure and the observed effects. Estimation of acceptable exposure levels should warrant a proper risk management. The aim of this review is to challenge the conception of CNTs/CNFs as a single, unified material entity and prompt the establishment of standardized hazard and exposure assessment methodologies able to properly feed risk assessment and management frameworks.

In plain English

This article is a review, not a new study. The authors gathered published research on carbon nanotubes (CNTs) and carbon nanofibers (CNFs). These are man-made materials that the article compares with asbestos fibers because they are similar in structure and biological behavior, especially in how long they persist in the body. The review covered animal and laboratory studies, studies of exposed workers, workplace air measurements and exposure limits. It reported that in 2014 the International Agency for Research on Cancer classified one multi-walled nanotube, MWCNT-7, as possibly carcinogenic to humans (group 2B). The authors described the animal evidence on mesothelioma as conflicting. Some rigid nanotubes caused mesothelioma when injected into the abdomen of animals, but tangled ones did not. Studies of workers linked exposure with subtle changes in body markers; the largest was a US cohort of 108 workers from 12 companies. A cited review found that 85% of field studies measured levels above the US NIOSH recommended limit of 1 µg/m3. The authors concluded that all CNTs and CNFs should be considered respiratory hazards and that exposure should be kept as low as possible.

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: Review of published animal, laboratory, worker and workplace-measurement studies on carbon nanotubes and carbon nanofibers; no new data were collected
• Who was studied: No study population of its own. The reviewed worker studies (Table 1) ranged in size from 9 exposed and 4 unexposed workers up to a US NIOSH cohort of 108 workers from 12 companies; animal and cell studies were also reviewed
• Exposure: Carbon nanotubes (single-walled SWCNTs and multi-walled MWCNTs) and carbon nanofibers (CNFs), mostly measured as elemental carbon in workplace air; asbestos itself was not studied but was used as a point of comparison
• Main results:
  • The review reported that in November 2014 the IARC Working Group classified MWCNT-7 as group 2B (possibly carcinogenic to humans), with limited evidence for other similar MWCNT types and no evidence of carcinogenicity for SWCNT
  • In cited animal studies, some rigid MWCNTs of a certain length and aspect ratio induced mesothelioma by intraperitoneal injection, while tangled MWCNTs did not; the authors described the overall evidence on mesothelial tumours as conflicting
  • From cited studies, the review stated that a length above 4 to 10 µm can almost be considered a threshold for CNT pathogenicity
  • The authors described the worker studies as cross-sectional and as suggesting an association between workplace exposure and subtle health effects (for example, changes in biomarkers in sputum, blood and exhaled breath)
  • Citing an earlier review, the authors reported that 85% of field studies found values above the NIOSH Recommended Exposure Level of 1 µg/m3
  • NIOSH recommended 1 µg/m3 elemental carbon as an 8-hour time-weighted average for respirable mass concentration. The authors noted that the listed exposure limits aim to protect against chronic respiratory outcomes such as lung fibrosis, not against cancer
• The authors' conclusion: The authors concluded that CNTs/CNFs are a varied class of materials rather than a single material. They also concluded that all types should be considered respiratory hazards, that occupational exposure should be kept as low as possible, and that a precautionary principle should be adopted until risk is better characterized.
• Limitations the authors state:
  • Epidemiological studies of workers are few and have many gaps in exposure assessment
  • Because overt health effects take a long time to appear, cohort studies with sufficient follow-up are needed; the existing worker studies are cross-sectional
  • Exposure measurements cover only the early stages of the materials' life cycle and a limited set of tasks
  • Some conclusions in the literature were reached at exposure concentrations unlikely to occur in workplaces

Topics

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Inflammation, biopersistence.

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