Sunscreen agents, also known as UV filters, are compounds that protect the skin from the harmful effects of ultraviolet UV radiation. They work through a combination of photochemical properties and mechanisms, which can be broadly categorized into two types: physical inorganic and chemical organic sunscreens.1. Physical inorganic sunscreens: These agents, such as titanium dioxide TiO2 and zinc oxide ZnO , are mineral-based and work by reflecting and scattering UV radiation. They form a protective barrier on the skin's surface, preventing UV rays from penetrating the skin. The photochemical properties of these agents include: a. High refractive index: Both TiO2 and ZnO have high refractive indices, which enable them to effectively reflect and scatter UV radiation. b. Wide absorption spectrum: Physical sunscreens can protect against both UVA 320-400 nm and UVB 280-320 nm radiation due to their wide absorption spectrum.2. Chemical organic sunscreens: These agents, such as oxybenzone, avobenzone, and octinoxate, are carbon-based compounds that work by absorbing UV radiation and converting it into a less harmful form of energy, such as heat. The photochemical properties and mechanisms of these agents include: a. Conjugated double bonds: Chemical sunscreens contain conjugated double bonds in their molecular structure, which allow them to absorb UV radiation effectively. The specific absorption wavelength depends on the structure and arrangement of these bonds. b. Excited state and energy dissipation: When a chemical sunscreen absorbs UV radiation, its molecules enter an excited state. The energy absorbed is then dissipated through various processes, such as internal conversion, vibrational relaxation, and fluorescence, which ultimately convert the energy into heat. c. Photostability: Some chemical sunscreens can degrade upon exposure to UV radiation, leading to a decrease in their effectiveness. To counter this, modern sunscreens often include photostabilizers or use more photostable compounds to maintain their protective properties.By utilizing these photochemical properties and mechanisms, sunscreen agents effectively prevent UV radiation from damaging the skin. This protection helps reduce the risk of sunburn, premature skin aging, and skin cancer caused by prolonged exposure to UV radiation.