Carbon nanotubes have distinctive characteristics, but their needle-like fibre shape has been compared to asbestos, raising concerns that widespread use of carbon nanotubes may lead to mesothelioma, cancer of the lining of the lungs caused by exposure to asbestos. Here we show that exposing the mesothelial lining of the body cavity of mice, as a surrogate for the mesothelial lining of the chest cavity, to long multiwalled carbon nanotubes results in asbestos-like, length-dependent, pathogenic behaviour. This includes inflammation and the formation of lesions known as granulomas. This is of considerable importance, because research and business communities continue to invest heavily in carbon nanotubes for a wide range of products under the assumption that they are no more hazardous than graphite. Our results suggest the need for further research and great caution before introducing such products into the market if long-term harm is to be avoided.

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Carbon nanotubes (CNT) are an important new class of technological materials that have numerous novel and useful properties. The forecast increase in manufacture makes it likely that increasing human exposure will occur, and as a result, CNT are beginning to come under toxicological scrutiny. This review seeks to set out the toxicological paradigms applicable to the toxicity of inhaled CNT, building on the toxicological database on nanoparticles (NP) and fibers. Relevant workplace regulation regarding exposure is also considered in the light of our knowledge of CNT. CNT could have features of both NP and conventional fibers, and so the current paradigm for fiber toxicology, which is based on mineral fibers and synthetic vitreous fibers, is discussed. The NP toxicology paradigm is also discussed in relation to CNT. The available peer-reviewed literature suggests that CNT may have unusual toxicity properties. In particular, CNT seem to have a special ability to stimulate mesenchymal cell growth and to cause granuloma formation and fibrogenesis. In several studies, CNT have more adverse effects than the same mass of NP carbon and quartz, the latter a commonly used benchmark of particle toxicity. There is, however, no definitive inhalation study available that would avoid the potential for artifactual effects due to large mats and aggregates forming during instillation exposure procedures. Studies also show that CNT may exhibit some of their effects through oxidative stress and inflammation. CNT represent a group of particles that are growing in production and use, and therefore, research into their toxicology and safe use is warranted.

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This review is concerned with evaluating the toxicity associated with human exposure to silver and gold nanoparticles (NPs), due to the relative abundance of toxicity data available for these particles, when compared to other metal particulates. This has allowed knowledge on the current understanding of the field to be gained, and has demonstrated where gaps in knowledge are. It is anticipated that evaluating the hazards associated with silver and gold particles will ultimately enable risk assessments to be completed, by combining this information with knowledge on the level of human exposure. The quantity of available hazard information for metals is greatest for silver particulates, due to its widespread inclusion within a number of diverse products (including clothes and wound dressings), which primarily arises from its antibacterial behaviour. Gold has been used on numerous occasions to assess the biodistribution and cellular uptake of NPs following exposure. Inflammatory, oxidative, genotoxic, and cytotoxic consequences are associated with silver particulate exposure, and are inherently linked. The primary site of gold and silver particulate accumulation has been consistently demonstrated to be the liver, and it is therefore relevant that a number of in vitro investigations have focused on this potential target organ. However, in general there is a lack of in vivo and in vitro toxicity information that allows correlations between the findings to be made. Instead a focus on the tissue distribution of particles following exposure is evident within the available literature, which can be useful in directing appropriate in vitro experimentation by revealing potential target sites of toxicity. The experimental design has the potential to impact on the toxicological observations, and in particular the use of excessively high particle concentrations has been observed. As witnessed for other particle types, gold and silver particle sizes are influential in dictating the observed toxicity, with smaller particles exhibiting a greater response than their larger counterparts, and this is likely to be driven by differences in particle surface area, when administered at an equal-mass dose. A major obstacle, at present, is deciphering whether the responses related to silver nanoparticulate exposure derive from their small size, or particle dissolution contributes to the observed toxicity. Alternatively, a combination of both may be responsible, as the release of ions would be expected to be greater for smaller particles.