Titanium dioxide (TiO2) is active in the UV region of the light spectra and is used as a photocatalyst in numerous applications. Photo-activated anatase TiO2 particles promote increased production of free radicals. This is a desirable property, although the potential toxicity of such photo-activated TiO2 particles on exposure of humans and the environment remains unknown. Therefore, we studied whether pre-irradiation of TiO2 particles with UV influences their cytotoxic and genotoxic potential. The TiO2 particles, as TiO2-A (<25 nm) and TiO2-B (>100 nm), were UV pre-irradiated (24 h) and tested for cytotoxic and genotoxic activities in human hepatoma HepC2 cells. Non-irradiated TiO2-A/B at 1.0-250 mu g/ml did not reduce viability of HepC2 cells, nor induce significant increases in DNA strand breaks: only TiO2-A induced significant increases in oxidative DNA damage. After UV pre-irradiation, both TiO2-A and TiO2-B reduced cell viability and induced significant increases in DNA strand breaks and oxidative DNA damage. This is the first study that shows that UV pre-irradiation of anatase TiO2 particles results in increased cytotoxic and genotoxic potential. This warrants further studies as it has important implications for environmental and human health risk assessment and preventive actions to limit human exposure. (C) 2011 Elsevier B.V. All rights reserved.