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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.

This critical review of the available human health safety data, relating to carbon nanotubes (CNTs), was conducted in order to assess the risks associated with CNT exposure. Determining the toxicity related to CNT exploitation is of great relevance and importance due to the increased potential for human exposure to CNTs within occupational, environmental and consumer settings. When this information is combined with knowledge on the likely exposure levels of humans to CNTs, it will enable risk assessments to be conducted to assess the risks posed to human health. CNTs are a diverse group of materials and vary with regards to their wall number (single and multi-walled CNTs are evident), length, composition, and surface chemistry. The attributes of CNTs that were identified as being most likely to drive the observed toxicity have been considered, and include CNT length, metal content, tendency to aggregate/agglomerate and surface chemistry. Of particular importance, is the contribution of the fibre paradigm to CNT toxicity, whereby the length of CNTs appears to be critical to their toxic potential. Mechanistic processes that are critical to CNT toxicity will also be discussed, with the findings insinuating that CNTs can exert an oxidative response that stimulates inflammatory, genotoxic and cytotoxic consequences. Consequently, it may transpire that a common mechanism is responsible for driving CNT toxicity, despite the fact that CNTs are a diverse population of materials. The similarity of the structure of CNTs to that of asbestos has prompted concern surrounding the exposure of humans, and so the applicability of the fibre paradigm to CNTs will be evaluated. It is also necessary to determine the systemic availability of CNTs following exposure, to determine where potential targets of toxicity are, and to thereby direct in vitro investigations within the most appropriate target cells. CNTs are therefore a group of materials whose useful exploitable properties prompts their increased production and utilization within diverse applications, so that ensuring their safety is of vital importance.