This special issue of Toxicology Letters presents new data on the toxicology of typical manufactured nanostructured materials. It includes results from on-going industry and academic projects on the (geno)toxicity of typical manufactured nanoparticles such as titanium dioxide and zinc oxide and newly engineered nanofibres such as carbon nanotubes. Attention is drawn to toxicologically relevant aspects such as characterisation, exposure, mode of action and translocation of these nanomaterials. A summary overviewand outlook concludes this issue. The papers included in this issue are based on presentations given at a symposium organised by the European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC). The symposium was held during the 37th Annual Meeting of EEMS, the European Environmental Mutagen Society in Basel, Switzerland, on Tuesday 11 September 2007. Toxicological investigations of nanomaterials often suffer from problems with stability of the test system and specific dosimetry. Early studies frequently lacked sufficient characterisation of the test material or appropriate control groups, i.e. in the absence of microparticle treated groups. Under such conditions, it is impossible to draw conclusions whether nanoparticle-mediated adverse effects are intrinsic to the test material or specific for particle size. Even in present studies, a change of test conditions might alter the results and influence the conclusions. This applies to in vitro models and to single or repeated dose studies in animals where route and mode of application can have different toxicological consequences. In all, to be useful for risk assessment, the model system has to be well characterised and representative of the chosen exposure situation. So far, there are few reports on the primary genotoxic effects of nanomaterials, but it is conceivable that some reactive nanomaterials may have genotoxic activity. It is presently unclear if the nano-scale dimensions could affect the possible genotoxicity. Depending on the inhaled dose, nanomaterials can induce significant inflammation and oxidative stress in the lung, which may trigger secondary genotoxic effects. Secondary genotoxicity mediated by toxicity might allow for the derivation of thresholds, as suggested for inert particles in general.
(Knaapen et al., 2004; Greim and Ziegler-Skylakakis, 2007).