Titanium dioxide and zinc oxide nanoparticles are being increasingly formulated in sunscreens. While the same compounds, in larger particle form, work by reflecting UV radiation, in nanoparticle form, they absorb UV radiation, resulting in photocatalysis, releasing reactive oxygen species. These reactive oxygen species are known to have the capability to alter DNA. Previous studies suggest that this photocatalytic process may not be significant, because the nanoparticles do not penetrate below the level of the stratum corneum. However, some recent studies suggest that nanoparticles may, under certain circumstances, breach that barrier. The majority of those studies have used animal skin models rather than human skin.