Photostability of UV Absorber Systems in Sunscreens†

Sunscreens are used to protect the human skin against harmful UV radiation. Today there is a trend toward higher sun protection factors (SPF) and better UVA protection. Methods for the assessment of SPF and UVA protection involve irradiation of the product, and the photostability properties of the sunscreen have an influence on its performance. Sunscreens often contain more than one UV filter. Thus it is important to understand the photostability properties of the complete system. The filter combinations used may exhibit destabilizing, stabilizing or inert interactions. For that reason, besides assessment of the properties of the single filters, photostabilities of binary filter combinations are investigated. Destabilization occurs when two UV absorbers undergo a chemical reaction after absorption of UV radiation. Stabilization may be achieved when the optical density of the system is very high, giving rise to a self‐protection effect of the sunscreen film. Photounstable UV absorbers may be additionally stabilized by employing triplet quenchers. Being aware of these mechanisms and applying them for specific UV filter combinations can help in designing efficient sunscreens.