防晒霜防晒原因
　　sunscreen and sunscreen
　　日光中的紫外线光谱是在100～400nm波长范围，可分为3个波段，即波长为100～290nm的短波紫外线（uvc），波长为290～320nm的中波紫外线（uvb）和波长为320～400nm的长波紫外线（uva）。不同波长的紫外线，穿透皮肤的深度是不同的。uvc被角质层反射和吸收，只有小部分穿透到表皮浅层。但由于uvc通过大气层时，大多被臭氧层所吸收而不能到达地球表面，因此基本不会对人体皮肤构成危害。uvb也是大部分被表皮吸收，小部分达到真皮浅层，是皮肤损伤的主要波段。它具有致癌性，但能被厚云层，衣服及玻璃阻断。uva小部分被表皮吸收，大部分可透入真皮，深达真皮中部。uva在夏天的中午到达地球表面的量要比uvb多100倍，可以穿透表皮达真皮而致真皮损伤，因此是主要的紫外线致病光谱。
　　the ultraviolet spectrum in sunlight is in the range of 100 to 400nm, and it can be pided into 3 bands, namely, short wave ultraviolet (uvc) with wavelength 100 to 290nm, ultraviolet b (uvb) with wavelength from 290 to 320nm, and long wave ultraviolet (uva) with wavelength of 320 to 400nm. the depth of the skin is different at different wavelengths of ultraviolet light. uvc is reflected and absorbed by the cuticle, and only a small portion penetrates to the superficial layer of the epidermis. but because uvc is mostly absorbed by the ozone layer when it passes through the atmosphere, it can't reach the earth's surface, so it does not harm the skin of human body. uvb is also mostly absorbed by the epidermis, and the small part reaches the superficial dermis, which is the main band of skin damage. it is carcinogenic, but can be blocked by thick clouds, clothes and glass. the small part of uva is absorbed by the epidermis, most of which can penetrate the dermis and reach the central part of the dermis. uva in the summer at noon at the earth's surface is uvb more than 100 times, can penetrate the table pida leather leather erzhi injury, so is the main disease causing ultraviolet spectrum.
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