Using a sensitive phi X174 RF plasmid DNA assay, free radical activity was detected at the surface of normal and ultrafine titanium dioxide (TiO2), environmental particles (PM-10), asbestos and a range of man-made fibres. There were differences in the amount of free radical activity that was detected, with ultrafine TiO2 being much more active than normal-sized TiO2; PM-10 also had substantial free radical activity. Amphibole asbestos samples were highly active, whilst man-made fibres were much less active than asbestos. For all of the particles, the DNA damage could be ameliorated by mannitol, showing that hydroxyl radicals were involved. The ability of particles to generate free radicals at or near their surface, and thereby impose oxidant stress in key target cells, could be central to determining their pathogenicity.