Photoprotection in changing times - UV filter efficacy and safety, sensitization processes and regulatory aspects

As excessive sun exposure is tightly associated with different pathological changes of the skin, for example premature skin ageing or the development of skin cancer, an appropriate protection of the skin against UV radiation is of particular importance. Sun protection products and UV filter substances have evolved continuously in the past few decades. New developments and improved technical conditions of production have led to increasingly effective and efficient products with broadband protection ability. Accordingly, legal requirements have also changed and expanded. Although certain trends exist to harmonize the regulation of sunscreens at a global level, there are still large differences how UV absorbers are approved, which testing methods are prescribed, and which general requirements sun protection products must fulfil. Modern UV filters provide efficient protection against UVA and UVB radiation, are heat and photostable, user-friendly, cost-effective, water resistant and non-toxic. As inorganic and organic UV filters are topically applied to the skin in relatively high concentrations (up to 25%), especially the assessment of their (photo)sensitization potential is of particular importance. Accordingly, skin sensitization is a key endpoint for the legally required safety assessment of cosmetic ingredients in Europe and many other countries. This review will summarize the current regulatory status of different approved UV filters, will describe their beneficial and adverse properties and will give an overview of how the efficacy of sunscreens can be evaluated. Finally, an insight into the basic mechanism of (photo)allergic reactions and existing skin sensitization test methods will be provided.