Microsized TiO2 and ZnO have been used as particulate sun-screen ingredients (average size approximately 0.1–10.0 μm) for more than 15 years. 47 Various microsized anatase and rutile TiO2 and wurtzite ZnO particles, coated and uncoated, have been utilized. The popularity of these inorganic white powders is due to a number of physicochemical properties described above. The UV attenuation results from both reflection and scattering of UV radiation and visible light (clarifying the opaqueness of these sunscreen formulations) and from UV absorption. UV attenuation properties of these two particles are complementary; TiO2 being primarily a UVB absorbing compound, while ZnO is more efficient in UVA absorption. Apart from size-related optical particle properties, the ability of particles to attenuate UV radiation is determined by the surrounding medium.

The size reduction of microsized ZnO and TiO2 increases UVB absorption of both particles at the expense of UVA-1 absorption, and the UV protection becomes unbalanced. A solution to this problem could be a combination of various ZnO microsized particles (∼200 nm or smaller to maintain transparency) and nanosized TiO2. Studies in this area reveal that further optimization results in the combination of two grades of ZnO particles (dispersed in cyclopentasiloxane or isononyl isononanoate) with slightly whitened 35 nm TiO2 NPs that provide better UVB protection. To guarantee the recommended balanced UVA/UVB protection, ZnO dispersions should preferably contain small nanosized and large microsized particles. According to this research, aggregated ZnO particles of 130 nm rather than the 20 nm primary particles influence UVA-1 protection.

The replacement of microsized TiO2 and ZnO particles by NPs ensures the cosmetically desired sunscreen transparency, but at the expense of broad UVA protection. Utilization of micro- and nanosized (20–200 nm) ZnO dispersions combined with nanosized 20–35 nm TiO2 particles may improve this situation. Skin exposure to the NP sunscreens leads to incorporation of TiO2 and ZnO NPs into the deepest SC layers and in the hair follicles that may serve as long-term reservoirs.