4.1.3 Daily skin sunscreen
1.Effect of ultraviolet radiation on skin
In daily life，ultraviolet radiation is the most common cause of skin damage.According to the different wavelengths，ultraviolet can be divided into three sections：long-wave ultraviolet（ultraviolet A，UVA），medium-wave ultraviolet（ultraviolet B，UVB）and shortwave ultraviolet（ultraviolet C，UVC）.Among them，the ultraviolet reaching the ground are about 95％of UVA and 5％of UVB，while UVC is basically absorbed by the ozone layer and cannot reach the ground.When exposed to the sun for a long time without taking any protective measures，the ultraviolet rays in the sun will cause varied degrees of damage to the skin.In the process of ultraviolet irradiation，free radicals and oxidation will be produced，which will damage the DNA，lipids and proteins of skin cells，resulting in skin erythema，skin blackening，skin photoaging，abnormal immune function and skin tumors.
The skin damage caused by ultraviolet radiation mainly includes the followings.
①Erythema reaction：Also called sun erythema，which is an acute inflammatory reaction.Erythema appears at 2-7 hours after sun exposure and reached the peak at 12-24 hours，which was mainly manifested as erythema and swelling in the sun site，and blisters occurred in severe cases，which could cause burning pain consciously.After a few days，the erythema disappeared and desquamation appeared.Patients with light skin are more likely to have erythema.The main light causing erythema reaction is UVB.
②Skin blackening：Also called skin tanning.It is characterized by clear gray-black spots with clear boundaries on the light site，and there are no self-conscious symptoms.According to the appearance time of pigment，it can be divided into immediate pigment darkening，persistent pigment darkening and delayed tanning.
a.Immediate pigment darkening is gray-black pigmentation that occurs during or immediately after irradiation and usually lasts only a few minutes to two hours.
b.With the increase of UV dose，persistent pigment darkening can overlap with erythema reaction，and then shows gray-black or dark brown.The peak is reached at 2-24 hours，which can last from several hours to several days.
c.Delayed tanning usually occurs within a few days after radiation and can last from weeks to months.Immediate pigment darkening and persistent pigment darkening are produced by oxidizing melanin particles and transporting them to keratinocytes，which are affected by UVA and visible light.Delayed tanning is caused by the active function of melanocytes and the synthesis of more melanin，which are affected by UVB，UVA and visible light.
③Skin photoaging：Long-term sunlight exposure will accelerate skin aging，which is mainly caused by UVA.It is characterized by rough skin，decreased elasticity，increased wrinkles and deepened pigmentation，accompanied by telangiectasia or hyperplasia and erythema formation.
④Abnormal immune function：The skin exposed to the ultraviolet can occur photoallergy，and abnormal function of local skin and systemic immune system can occur at the same time.Immune changes caused by light involve immunoreactive cells，cytokines，complements and the interaction between these substances，which can cause a variety of photolinear diseases and light-exacerbated skin diseases.
⑤Skin tumors：UVB irradiation can cause nuclear base structure changes.UVA induces cells to produce reactive oxygen clusters，which leads to abnormal structure of cell membrane，DNA degeneration and destruction of lipids and proteins.As a result，malignant skin diseases occur，including photolinear keratosis，basal cell carcinoma，squamous cell carcinoma and melanoma.(https://www.daowen.com)
2.Common sunscreen measures and products
In 2002，ultraviolet index（UVI）was put forward by the World Health Organization（WHO），which is used to indicate the ultraviolet intensity of sunlight.And the higher UVI induces the greater damage of sunlight to the skin and eyes.Generally，noon is the highest period for UVI，and late spring and summer are the higher seasons for UVI.UVI increased with the increase of altitude.In addition，seaside beaches，snow，wall or curtain glass of urban high-rise buildings，car windowpanes and hardened floors by the asphalt or cement all reflect ultraviolet and increase UVI.Daily sunscreen measures mainly include shielding sunscreen and the use of sunscreen agents.
（1）Shielding sunscreen：Shielding sunscreen refers to the use of sunscreen umbrellas，sunscreen mirrors，sunscreen caps，sunscreen clothing or masks and other shielding objects to block the exposed parts of the body.The shield can block ultraviolet rays to achieve sunscreen.High-density weaving，darker colors and sunscreen coatings can enhance the shielding ability of the shielding to ultraviolet rays，which means better sunscreen.If a sunscreen hat is used，the edge length of the brim should be above 7.5 cm for requiring a better sunscreen.It is recommended to buy fabric products marked with UFP＞25 dyn and UVA transmittance＜5％（Table 4-1）.Sunshades that cover all UV and minimize the penetration of blue and purple light are recommended.
Table 4-1 Ultraviolet radiation protection index of fabric（Skin Sun Protection Expert Consensus 2017）
（2）The use of sunscreen agents：Sunscreen is a substance that absorbs，reflects or scatters light to protect the skin from specific UV damage.At present，the sunscreen products on the market are mainly divided into three categories：organic sunscreen based on ultraviolet absorbers，inorganic sunscreen based on ultraviolet shielding agents and antioxidants that indirectly play a role in sunscreen.
①Organic sunscreen：It is also called chemical UV absorbent.Most organic sunscreen agents contain aromatic compounds，where the special groups on the benzene ring can affect the spectral properties of sunscreen agents.Generally，this kind of substance can selectively absorb UV and result in sunscreen.The sunscreen agents that absorb UVB mainly include salicylates and their derivatives and cinnamates；butyl methoxydibenzoylmethane is the main absorbent of UVA；benzophenone and its derivatives are both absorbable.Organic sunscreen is favorable for the light and thin texture and excellent sense of transparency.However，the photostability oftraditional organic sunscreen is not as stable as that of inorganic sunscreen.Moreover，organic sunscreen is easy to be absorbed through the skin and has a certain degree of sensitization，which may lead to contact sensitization and photosensitization.In recent years，a large number of new organic sunscreen agents have been put on the market，which not only overcome the shortcomings of traditional organic sunscreen agents，but also significantly improve the solubility and photostability of sunscreen agents through isomerization and micronization，and these are not easy to be absorbed through the skin.The new organic sunscreen agents have been becoming favored by the market for their safety and effectiveness.
②Inorganic sunscreen：It is also called physical UV shielding agent.This kind of sunscreen reflect and scatter sunlight，such as titanium dioxide（TiO2），and zinc oxide（ZnO）.The main principle is to form a uniform protective layer，which can reflect，refract and scatter ultraviolet，thus reducing the damage of ultraviolet to the skin.In recent years，it has been found that inorganic sunscreen particles have semiconductor-like properties，and electron transition process in the atoms can selectively absorb ultraviolet，resulting in sunscreen.Inorganic sunscreen is favored for wide sunscreen spectrum and relatively light stability.Meanwhile，this kind of sunscreen is not sensitized.Thus，it is suitable for people with sensitive skin.However，the disadvantage of this kind of sunscreen is that it is not easy to apply，and excessive use is easy to clog pores and cause new skin diseases.
③Antioxidants：This kind of sunscreen has no direct absorption or reflection effect on sunlight.Yet，addition of it to cosmetics can improve the antioxidant capacity of the skin，thus indirectly play the role of sunscreen.These compounds mainly include plant extracts such as vitamin C/E，β-carotene，metallothionein，superoxide dismutase（SOD），anthocyanin，and tetrahydromethyl pyrimidine carboxylic acid（Ectoin）.Licorice，aloe，green tea，panax notoginseng and grape seeds also have antioxidants.
Generally speaking，all kinds of sunscreen agents at present have their own advantages and disadvantages.In order to cover both UVB and UVA，and take into account both safety and applicability，most sunscreen cosmetics are compounded of raw materials with different mechanisms of action.