Hazards of TiO2 and amorphous SiO2 nanoparticles

TiO₂ and amorphous SiO₂ nanoparticles have been described as ‘safe’, ‘non-toxic’ and ‘environment friendly’ in scientific literature. However, though toxicity data are far from complete, there is evidence that these nanoparticles are hazardous. TiO₂ nanoparticles have been found hazardous to humans on inhalation, ingestion and dermal exposure. Ecotoxicity at levels of TiO₂ nanoparticles which are expected in the environment has also been found. Amorphous SiO₂ nanoparticles appear to be hazardous to humans on inhalation and ingestion and there is some evidence for ecotoxicity of amorphous SiO₂ nanoparticles. A main, though not the only, mechanism underlying the hazards of SiO₂ and TiO₂ nanoparticles may be the generation of reactive oxygen species. In view of the lack of scientific data pertinent to quantification of hazard and risk, a precautionary approach to production and usage of SiO₂ and TiO₂ nanoparticles has been advocated. Options for hazard reduction, such as coatings for TiO₂ nanoparticles, functionalization for amorphous SiO₂ nanoparticles and binding of nanoparticles to substrates, and risk reduction, including containment and membrane filtration, are discussed.