Titanium dioxide and zinc oxide nanoparticles in sunscreens: Focus on their safety and effectiveness

Sunscreens are used to provide protection against adverse effects of ultraviolet (UV)B (290-320 nm) and UVA (320-400 nm) radiation. According to the United States Food and Drug Administration, the protection factor against UVA should be at least one-third of the overall sun protection factor. Titanium dioxide (TiO 2) and zinc oxide (ZnO) minerals are frequently employed in sunscreens as inorganic physical sun blockers. As TiO 2 is more effective in UVB and ZnO in the UVA range, the combination of these particles assures a broad-band UV protection. However, to solve the cosmetic drawback of these opaque sunscreens, microsized TiO 2 and ZnO have been increasingly replaced by TiO 2 and ZnO nanoparticles (NPs) (,100 nm). This review focuses on significant effects on the UV attenuation of sunscreens when microsized TiO 2 and ZnO particles are replaced by NPs and evaluates physicochemical aspects that affect effectiveness and safety of NP sunscreens. With the use of TiO 2 and ZnO NPs, the undesired opaqueness disappears but the required balance between UVA and UVB protection can be altered. Utilization of mixtures of micro- and nanosized ZnO dispersions and nanosized TiO 2 particles may improve this situation. Skin exposure to NP-containing sunscreens leads to incorporation of TiO 2 and ZnO NPs in the stratum corneum, which can alter specific NP attenuation properties due to particle-particle, particle-skin, and skin-particle-light physicochemical interactions. Both sunscreen NPs induce (photo)cyto- and genotoxicity and have been sporadically observed in viable skin layers especially in case of long-term exposures and ZnO. Photocatalytic effects, the highest for anatase TiO 2, cannot be completely prevented by coating of the particles, but silica-based coatings are most effective. Caution should still be exercised when new sunscreens are developed and research that includes sunscreen NP stabilization, chronic exposures, and reduction of NPs' free-radical production should receive full attention.