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UV Filters and Their Distribution on the Skin through Safe, Non-Penetrating Vehicles PANAGOULA PAVLOU, ANGELIKI SIAMIDI, MARILENA VLACHOU, and ATHANASIA VARVARESOU , Division of Aesthetics and Cosmetic Science, Department of Biomedical Sciences, University of West Attica, Egaleo 12243, Greece (P.P., A.V.), Section of Pharmaceutical Technology, Department of Pharmacy, School of Health Sciences, National and Kapodistrian University of Athens, Athens 15784, Greece (P.P., A.S., M.V.), Laboratory of Chemistry-Biochemistry- Cosmetic Science, Department of Biomedical Sciences, University of West Attica, Egaleo 12243, Greece (P.P., A.V.) Accepted for publication February 21, 2021. Synopsis The effects of ultraviolet (UV) radiation trigger human skin reaction, which can result in erythema, photoaging, and/or skin cancer. Sunscreens play an important role against the negative effects of UV radiation on the human skin. However, they should satisfy certain criteria, with the main one being photostability, to avoid the formation of health-threatening reactive intermediates. It has to be kept in mind, however, that photo-stable UV fi lters have the undesirable propensity to transfer energy to molecular oxygen and generate the very reactive singlet oxygen. They should also be well tolerated, while at the same time, they should not permeate into the skin and cause toxic effects. Thus, there is an ongoing need to develop effective and safe non-penetrating sunscreen formulations. The search for innovative active substances, effi cacious combinations, and the design of vehicles or carriers has led to the implementation of advanced delivery systems. This study intended to review the commonly used UV radiation thwarting agents (organic and inorganic UV fi lters), compile the relevant toxicity studies, evaluate their margin of safety, and assess the current situation on innovative sunscreen formulations. INTRODUCTION The ultraviolet (UV) radiation causes human skin reactions, which could lead to erythema, photoaging, and/or skin cancer (1). Against the negative effects of UV radiation on the human skin, sunscreens play an important role. Their safety depends on whether they cause irritation or sensitization, or on their ability to penetrate into the skin (cutaneous permeation). In this case, systemic toxicity, by allowing the product to pass into the bloodstream, could develop (2). The sunscreens should also be photostable, to avoid UV protection loss and the formation of health-threatening reactive intermediates while Address all correspondence to Panagoula Pavlou at gipav@pharm.uoa.gr J. Cosmet. Sci., 72, 298–324 (May/June 2021) 298