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

Threes G Smijs^1–3, Stanislav Pavel^4
1Faculty of Science, Open University in The Netherlands, Rotterdam, The Netherlands; ²University of Leiden, Leiden Amsterdam Center for Drug Research, Leiden, The Netherlands; ³Erasmus MC, Center for Optical Diagnostics and Therapy, Rotterdam, The Netherlands; ⁴Charles University, Faculty of Medicine, Department of Dermatology, Pilsen, Czech Republic

Abstract: 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 (TiO₂) and zinc oxide (ZnO) minerals are frequently employed in sunscreens as inorganic physical sun blockers. As TiO₂ 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 TiO₂ and ZnO have been increasingly replaced by TiO₂ and ZnO nanoparticles (NPs) (<100 nm). This review focuses on significant effects on the UV attenuation of sunscreens when microsized TiO₂ and ZnO particles are replaced by NPs and evaluates physicochemical aspects that affect effectiveness and safety of NP sunscreens. With the use of TiO₂ 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 TiO₂ particles may improve this situation. Skin exposure to NP-containing sunscreens leads to incorporation of TiO₂ 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 TiO₂, 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.

Keywords: skin barrier, TiO₂, ZnO, nanoparticles, physicochemical, scattering, blue shift, UV-radiation, (photo) toxicity, cancer