Together is better: How mixtures of organic filters mitigate the photocatalytic activity of TiO 2 and ZnO

The sunscreen market has witnessed a significant shift towards mineral filters, particularly zinc oxide (ZnO) and titanium dioxide (TiO ₂ ), due to their lower toxicological profile and lack of endocrine-disrupting properties associated with some organic filters. There is, however, an important knowledge gap about the potential impact of ZnO and TiO ₂ on the (unwanted) photocatalytic degradation of organic filters in sunscreen formulations, which would significantly decrease their protecting efficacy towards the ultraviolet radiation. In this study, we investigate the degradation kinetics of six recurrent organic filters in the presence and absence of either ZnO or TiO ₂ . The obtained results confirm a Langmuir-Hinshelwood mechanism for the degradation process, with a noteworthy contribution from ZnO and TiO ₂ acting as photocatalysts. Interestingly, the study also reveals that a mixture of organic filters can modulate the photocatalytic activity of the metal oxides, emphasizing the need for further investigation into optimizing sunscreen formulations in terms of both safety and effectiveness, and opens promising strategies for developing sunscreens with enhanced stability and protection.