A review of exposure and toxicological aspects of carbon nanotubes, and as additives to fire retardants in polymers

Our plain-English summary of A review of exposure and toxicological aspects of carbon nanotubes, and as additives to fire retardants in polymers, by A. Christou, A.A. Stec, W. Ahmed, et al (Critical Reviews in Toxicology, 2016).

The study

• Authors: A. Christou, A.A. Stec, W. Ahmed, K. Aschberger, V. Amenta
• Journal: Critical Reviews in Toxicology
• Published: 2016;46(1):74-95
• Type of article: Review
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Carbon nanotubes (CNTs) have attracted considerable interest due to their unique physical, chemical, optical and electrical properties opening avenues for a large number of industrial applications. They have shown potential as fire retardant additives in polymers, reducing heat release rate and increasing time to ignition in a number of polymers. Relevant work on the types, properties and applications has been reviewed particularly considering their application in fire situations. There are concerns over the health risks associated with CNTs and many papers have likened CNTs to the health problems associated with asbestos. There are contradictions relating to the toxicity of CNTs with some papers reporting that they are toxic while others state the opposite. Directly comparing various studies is difficult because CNTs come in many combinations of size, type, purity levels and source. CNTs can potentially be released from polymers during the combustion process where human exposure may occur. While this review has shed some light regarding issues relating to toxicity under different fire scenarios much more thorough work is needed to investigate toxicity of CNTs and their evolution from CNT-polymer nanocomposites in order to reach firm conclusions.

In plain English

This article was a review, not an original study. The authors reviewed published work on carbon nanotubes (CNTs): their types, manufacture and properties, their use as fire-retardant additives in polymers, animal toxicity studies, workplace exposure, and release of CNTs from CNT–polymer materials through sanding, weathering and burning. The authors summarised the animal studies, mostly in mice and rats, as showing that inhaled free-standing CNTs can cause lung inflammation and granulomas, and in some cases fibrosis and cancer. In one study they discussed, long multi-walled CNTs injected into the abdominal cavity of mice gave responses similar to amosite asbestos fibres. The review noted that no human cancer data were available. It also reported that IARC had classified one product, MWCNT-7, as possibly carcinogenic to humans (group 2B). The authors found very little evidence of free CNTs being released from consumer products by mechanical forces, and few studies of release in fires. They concluded that typical composites are not likely to release CNTs in quantities that would have toxic effects, but that further work on fire scenarios is needed.

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: Narrative literature review (no original data; no search method or number of studies reported)
• Who was studied: Published studies on carbon nanotubes: mainly animal toxicity studies in mice and rats, plus workplace and laboratory exposure measurements and studies of CNT release from polymer composites
• Exposure: Carbon nanotubes (single-walled, double-walled and multi-walled), both free-standing and embedded in polymer composites, including CNTs released by mechanical abrasion, UV weathering and combustion
• Main results:
  • The authors summarised the reviewed animal studies as showing that inhaled free-standing CNTs can cause lung inflammation and granulomas, and in some cases fibrosis and cancer
  • In a reviewed pilot study (Poland et al. 2008), long multi-walled CNTs injected into the abdominal cavity of mice produced a response similar to amosite asbestos fibres
  • No human cancer data were available. IARC had classified one brand, MWCNT-7, in group 2B (possibly carcinogenic to humans)
  • The reviewed studies gave very little evidence that free-standing CNTs are released from consumer products when mechanical forces are applied
  • In the reviewed UV weathering studies, a CNT network remained on the material and no release into the environment was seen
  • Few studies had examined CNT release from burning polymers. In one reviewed study (Bouillard et al. 2013), on burning a plastic containing multi-walled CNTs, free multi-walled CNTs were detected in the collected samples
• The authors' conclusion: The authors concluded that typical CNT–polymer composites contain very little CNT and are not likely to release it in amounts large enough to have toxic effects, but that further work on release in different fire scenarios is required
• Limitations the authors state:
  • Results in rats and mice are difficult to translate to humans because their respiratory systems differ greatly in size
  • Toxicity studies used free-standing CNTs, and little work exists on CNTs released from fire-retardant polymers, so direct comparison is difficult
  • Studies are hard to compare because CNTs vary in size, type, purity and source
  • There are no validated sampling and analysis methods specific to engineered nanomaterials such as CNTs

Related studies

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• Review of carbon nanotubes toxicity and exposure--appraisal of human health risk assessment based on open literature — K. Aschberger, H.J. Johnston, et al. (2010)
• Exposure assessment of carbon nanotube manufacturing workplaces — J.H. Lee, S.B. Lee, et al. (2010)
• Inhalation of rod-like carbon nanotubes causes unconventional allergic airway inflammation — E.M. Rydman, M. Ilves, et al. (2014)
• Length-dependent pleural inflammation and parietal pleural responses after deposition of carbon nanotubes in the pulmonary airspaces of mice — F.A. Murphy, C.A. Poland, et al. (2013)
• In vitro and in vivo genotoxic effects of straight versus tangled multi-walled carbon nanotubes — J. Catalán, K.M. Siivola, et al. (2016)

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