Sunscreen is mainly UV protection, UVA can cause skin darkening, aging, UVB can cause sunburn, skin tumors, so an ideal sunscreen should be as far as possible while blocking UVA He UVB. Sunscreen inside the two main components: a major component is the physical covering substances, such as titanium dioxide; zinc oxide, it is a very small particles, mixed into the cream inside, applied to the skin above It appears white and can form a protective film, reflecting the UV rays out of the whole. Another class of ingredients is the chemical sunscreen substances, often containing organic sunscreen agents, such as azobenzene, cinnamic acid esters, etc., can absorb the energy of ultraviolet rays and converted into heat to be dissipated. The size of its role in sun protection UVB can be measured by SPF, and the ability to protect against UVA is mostly expressed in PA value in China. Yang Shuxia, Department of Dermatology and Venereology, Peking University First Hospital Both have their advantages and disadvantages, chemical sunscreens feel delicate and light, but are prone to skin sensitivity or irritation. The main ingredients of physical sunscreens are titanium dioxide and zinc oxide, which can only be formulated as an oily cream because of their physical and chemical properties, so they are greasy, heavy and uncomfortable after use, but the protection capacity of such physical sunscreens is relatively high, and they are very safe, non-toxic and non-irritating, especially for people with sensitive skin and children. Most current sunscreens are a mixture of two ingredients, taking the best of both worlds to achieve a refreshing and non-oily, and gentle, non-irritating quality. A particularly refreshing sunscreen is certainly not as effective as a greasy sunscreen. Because in the effect of sunscreen, physical coverage is playing a very important role, so the sunscreen index is high must be more greasy, if very refreshing, unless there are many chemical additives, but its irritation or sensitivity is relatively high.