Review on the application of inorganic UV filters in sunscreens: Mechanisms, evaluation methods, toxicity, and safety enhancements

The increasing intensity of UV radiation in sunlight due to the thinning of the ozone layer induces various health risks, especially, the ones that are related to skin diseases. In addition, the increase in life quality has led to the elevation of beauty standards. Therefore, the development and utilization of sunscreen have become a crucial aspect as they can effectively provide protection against the harmful radiation from the sun and hinder aesthetic-related disorders derived from prolonged exposure to sunlight. With recent advances in material technology, the applications of titanium dioxide (TiO₂) and zinc oxide (ZnO) as UV filters in sunscreens have been widely adopted, therefore, they are selected as the main focal point of this review. Insights into the mechanisms of different types of sun protection exhibited by both conventional microparticles and novel nanoparticles are also provided. In addition, various methods used to evaluate the protection performance of sun protection are also discussed in detail. The review further critically evaluates the safety issues associated with TiO₂ and ZnO nanoparticles, as well as the potential risks, and proposes strategies to improve their safety when they are used in sunscreen.