During the last few years, research on toxicologically relevant properties of engineered nanoparticles has increased tremendously. A number of international research projects and additional activities are ongoing in the EU and the US, nourishing the expectation that more relevant technical and toxicological data will be published. Their widespread use allows for potential exposure to engineered nanoparticles during the whole lifecycle of a variety of products. When looking at possible exposure routes for manufactured Nanoparticles, inhalation, dermal and oral exposure are the most obvious, depending on the type of product in which Nanoparticles are used. This review shows that (1) Nanoparticles can deposit in the respiratory tract after inhalation. For a number of nanoparticles, oxidative stress-related inflammatory reactions have been observed. Tumour-related effects have only been observed in rats, and might be related to overload conditions. There are also a few reports that indicate uptake of nanoparticles in the brain via the olfactory epithelium. Nanoparticle translocation into the systemic circulation may occur after inhalation but conflicting evidence is present on the extent of translocation. These findings urge the need for additional studies to further elucidate these findings and to characterize the physiological impact. (2) There is currently little evidence from skin penetration studies that dermal applications of metal oxide nanoparticles used in sunscreens lead to systemic exposure. However, the question has been raised whether the usual testing with healthy, intact skin will be sufficient. (3) Uptake of nanoparticles in the gastrointestinal tract after oral uptake is a known phenomenon, of which use is intentionally made in the design of food and pharmacological components. Finally, this review indicates that only few specific nanoparticles have been investigated in a limited number of test systems and extrapolation of this data to other materials is not possible. Air pollution studies have generated indirect evidence for the role of combustion derived nanoparticles (CDNP) in driving adverse health effects in susceptible groups. Experimental studies with some bulk nanoparticles (carbon black, titanium dioxide, iron oxides) that have been used for decades suggest various adverse effects. However, engineered nanomaterials with new chemical and physical properties are being produced constantly and the toxicity of these is unknown. Therefore, despite the existing database on nanoparticles, no blanket statements about human toxicity can be given at this time. In addition, limited ecotoxicological data for nanomaterials precludes a systematic assessment of the impact of Nanoparticles on ecosystems.

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.

This review considers the molecular toxicology of combustion-derived nanoparticles (CDNP) following inhalation exposure. CDNP originate from a number of sources and in this review we consider diesel soot, welding fume, carbon black and coal fly ash. A substantial literature demonstrates that these pose a hazard to the lungs through their potential to cause oxidative stress, inflammation and cancer; they also have the potential to redistribute to other organs following pulmonary deposition. These different CDNP show considerable heterogeneity in composition and solubility, meaning that oxidative stress may originate from different components depending on the particle under consideration. Key CDNP-associated properties of large surface area and the presence of metals and organics all have the potential to produce oxidative stress. CDNP may also exert genotoxic effects, depending on their composition. CDNP and their components also have the potential to translocate to the brain and also the blood, and thereby reach other targets such as the cardiovascular system, spleen and liver. CDNP therefore can be seen as a group of particulate toxins unified by a common mechanism of injury and properties of translocation which have the potential to mediate a range of adverse effects in the lungs and other organs and warrant further research.

Engineered nanoparticles (NPs) are widely used in different technologies but their unique properties might also cause adverse health effects. In reviewing recent in vitro and in vivo genotoxicity studies we discuss potential mechanisms of genotoxicity induced by NPs. Various factors that may influence genotoxic response, including physico-chemical properties and experimental conditions, are highlighted. From 4346 articles on NP toxicity, 112 describe genotoxicity studies (94 in vitro, 22 in vivo). The most used assays are the comet assay (58 in vitro, 9 in vivo), the micronucleus assay (31 in vitro, 14 in vivo), the chromosome aberrations test (10 in vitro, 1 in vivo) and the bacterial reverse mutation assay (13 studies). We describe advantages and potential problems with different methods and suggest the need for appropriate methodologies to be used for investigation of genotoxic effects of NPs, in vitro and in vivo.

Due to the rapid development of a diverse array of nanoparticles, used in a wide variety of products, there are now many international activities to assess the potential toxicity of these materials. These particles are developed due to properties such as catalytic reactivity, high surface area, light emission properties, and others. Such properties have the potential to interfere in many well-established toxicity testing protocols. This article outlines some of the most frequently used assays to assess the cytotoxity and biological reactivity of nanoparticles in vitro. The article identifies key issues that need to be addressed in relation to inclusion of relevant controls, assessing particles for their ability to interfere in the assays, and using systematic approaches to prevent misinterpretation of data. The protocols discussed range from simple cytotoxicity assays, to measurement of reactive oxygen species and oxidative stress, activation of proinflammatory signaling, and finally genotoxicity. The aim of this review is to share knowledge relating to nanoparticle toxicity testing in order to provide advice and support for guidelines, regulatory bodies, and for scientists in general.