Review of carbon nanotubes toxicity and exposure--appraisal of human health risk assessment based on open literature

Our plain-English summary of Review of carbon nanotubes toxicity and exposure--appraisal of human health risk assessment based on open literature, by K. Aschberger, H.J. Johnston, V. Stone, et al (Critical Reviews in Toxicology, 2010).

The study

• Authors: K. Aschberger, H.J. Johnston, V. Stone, R.J. Aitken, S.M. Hankin, S.A. Peters, C.L. Tran, F.M. Christensen
• Journal: Critical Reviews in Toxicology
• Published: 2010;40(9):759-90
• Type of article: Review
• Cited by: 128 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Carbon nanotubes (CNTs) possess many unique electronic and mechanical properties and are thus interesting for numerous novel industrial and biomedical applications. As the level of production and use of these materials increases, so too does the potential risk to human health. This study aims to investigate the feasibility and challenges associated with conducting a human health risk assessment for carbon nanotubes based on the open literature, utilising an approach similar to that of a classical regulatory risk assessment. Results indicate that the main risks for humans arise from chronic occupational inhalation, especially during activities involving high CNT release and uncontrolled exposure. It is not yet possible to draw definitive conclusions with regards the potential risk for long, straight multi-walled carbon nanotubes to pose a similar risk as asbestos by inducing mesothelioma. The genotoxic potential of CNTs is currently inconclusive and could be either primary or secondary. Possible systemic effects of CNTs would be either dependent on absorption and distribution of CNTs to sensitive organs or could be induced through the release of inflammatory mediators. In conclusion, gaps in the data set in relation to both exposure and hazard do not allow any definite conclusions suitable for regulatory decision-making. In order to enable a full human health risk assessment, future work should focus on the generation of reliable occupational, environmental and consumer exposure data. Data on toxicokinetics and studies investigating effects of chronic exposure under conditions relevant for human exposure should also be prioritised.

In plain English

This article was a review, not a new experiment. The authors gathered publicly available exposure and toxicity data on carbon nanotubes (single-walled, double-walled and multi-walled) up to June 2010. They used these data to attempt a human health risk assessment, following European chemical-regulation guidance that they noted was not developed for nanomaterials. From two 90-day rat inhalation studies of specific multi-walled nanotubes, they derived indicative no-effect levels for workers of 1 and 2 µg/m3 for long-term inhalation. A separate single 6-hour rat inhalation study gave 150 µg/m3 for short-term inhalation. The long-term levels were much lower than most of the medium and high workplace exposure values reported, which the authors read as pointing to a possible risk for workers. Estimated environmental air levels were much lower than the 0.25 µg/m3 level derived for the general public. In the reviewed animal studies, long, straight multi-walled nanotubes injected into the abdominal cavity or scrotum of mice and rats could cause mesothelioma. No cancer study by inhalation existed. The authors judged the genotoxic potential inconclusive. They concluded that, because of data gaps, their results should not be used for regulatory decisions.

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 literature with a risk assessment appraisal modelled on European chemical regulation (REACH / Existing Substances) methodology
• Who was studied: No new subjects. Published exposure studies (occupational inhalation exposure measured in eight workplaces and laboratories) and animal and cell toxicity studies of carbon nanotubes, available as of June 2010
• Exposure: Carbon nanotubes (single-walled, double-walled and multi-walled) in occupational, consumer and environmental settings; medical applications excluded
• Main results:
  • The acute worker inhalation level was based on a LOAEC of 11 mg/m3 for multi-walled nanotubes from a single 6-hour rat inhalation study (Ellinger-Ziegelbauer and Pauluhn, 2009)
  • Indicative no-effect level for acute worker inhalation of multi-walled carbon nanotubes: 150 μg/m3
  • Indicative no-effect levels for chronic worker inhalation, from two 90-day rat studies: 2 μg/m3 for MWCNT1 and 1 μg/m3 for MWCNT2. For the general public, the level was 0.25 μg/m3
  • The chronic worker levels (1 and 2 µg/m3) were much lower than most of the medium and high occupational exposure values identified, pointing to a possible risk for workers chronically inhaling multi-walled nanotubes
  • Calculated environmental air concentrations were much lower than the general-public chronic level of 0.25 µg/m3, so no elevated risk was expected under those conditions
  • For dermal exposure, an estimated worker exposure reduced by 90% with gloves (1.2 mg/person) was lower than the dermal no-effect levels, except the chronic level in scenario 2
  • From the reviewed studies, long, straight multi-walled nanotubes could induce mesotheliomas after intraperitoneal or intrascrotal injection in mice and rats
  • In one reviewed mouse study (Poland et al., 2008), long-fibre multi-walled nanotube samples caused inflammatory responses, granulomas and foreign body giant cells comparable to long-fibre amosite asbestos
  • No carcinogenicity study using inhalation was available
  • The genotoxic potential of carbon nanotubes was judged inconclusive
• The authors' conclusion: The authors concluded that long, straight multi-walled nanotubes are suspected to pose a mesothelioma risk similar to asbestos, but that this had not been proven under physiologically relevant conditions such as inhalation. They also concluded that, because of uncertainties in exposure and hazard data, their results should not be used for regulatory decision-making.
• Limitations the authors state:
  • The data related to different nanotube types, and in some original papers the material characterisation was unclear or incomplete
  • The REACH guidance used was not developed for the specific characteristics of nanomaterials
  • The key studies tested relatively short multi-walled nanotubes; no study tested long, straight ones under physiologically relevant conditions
  • Exposure measurements covered few situations over short periods, which made their representativeness hard to judge
  • The dermal no-effect levels rested on only one short-term, non-guideline study
  • No conclusions on carcinogenic or genotoxic effects could be made from the data used

Topics

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

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