UV Filters and Their Distribution on the Skin through Safe, Non-Penetrating Vehicles

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 filters 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, efficacious 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 filters), compile the relevant toxicity studies, evaluate their margin of safety, and assess the current situation on innovative sunscreen formulations.