In the wavelength range of 100-400nm, the ultraviolet spectrum in sunlight can be divided into three bands, namely, short-wave ultraviolet (UVC) with wavelength of 100-290nm, medium-wave ultraviolet (UVB) with wavelength of 290-320nm and long-wave ultraviolet (UVA) with wavelength of 320-400nm.
Ultraviolet rays with different wavelengths penetrate the skin at different depths. UVC is reflected and absorbed by the stratum corneum, and only a small part penetrates into the superficial layer of epidermis. However, when UVC passes through the atmosphere, it is mostly absorbed by the ozone layer and cannot reach the earth's surface, so it is basically harmless to human skin. UVB is also mostly absorbed by epidermis, and a small part reaches the superficial dermis, which is the main wave band of skin damage. It is carcinogenic, but it can be blocked by thick clouds, clothes and glass. A small part of UVA is absorbed by epidermis, and most of UVA can penetrate dermis and reach the middle of dermis. The amount of UVA reaching the earth's surface at noon in summer is 100 times that of UVB, which can penetrate the epidermis to the dermis and cause dermal damage, so it is the main ultraviolet pathogenic spectrum.