Photostabilization of Organic UV Filters: Toward Improved Sunscreen Formulation

Ultraviolet (UV) radiation from sunlight is a major cause of skin damage and adverse health effects. To prevent the onset of diseases and other undesirable outcomes, cosmetic preparations such as sunscreens are commonly used. However, their formulation and development involve several challenges, including the photoinstability of certain organic filters. In this study, the effect of three cosmetic-grade solvents on four lipophilic UV filters was evaluated, as well as the effect of three solvents on one water-soluble filter, with regard to spectral behavior and their photostability. In addition, 25 oil-in-water (O/W) emulsions were prepared following an extreme vertices mixture design, combining five variable liposoluble UV filters, one fixed water-soluble UV filter, one antioxidant, and one solvent. Physical stability was evaluated first, followed by photochemical behavior. The responses evaluated included the sun protection factor (SPF), UVA protection factor (UVAPF), and critical wavelength (λ c ) before and after irradiation under stress conditions. The findings demonstrated that chemical photostabilization mechanisms of organic UV filters, such as the synergistic combination of lipophilic UV filters and the addition of antioxidants, have a significant impact on the development of sunscreen formulation strategies. These strategies involve assessing the individual behavior of UV filters in solvents, followed by evaluation within a mixture design, thereby ensuring that SPF and UVAPF values remain within effective photoprotection ranges, even under high irradiation stress conditions.