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When it is exposed to the sun, the skin is capable of reflecting some of the ultraviolet rays it receives. The remainder is absorbed, going through the surface and penetrating the epidermis and the dermis.
There are three types of ultraviolet rays (UV): UVA, UVB and UVC.
In the short term, UVA does not have any harmful effects. They react with the pigments which are already present in the skin, which leads to a colouration of the skin which is almost immediate, ‘tanning’, which does not last.
However, the dangers caused by UVA rays are numerous because they can reach the dermis. There they can trigger the production of free radicals, these molecules which attack the surface of healthy cells or even attack their DNA. They also generate cancerous cells and are considered to be a risk factor for melanomas.
UVA rays also affect the molecules of collagen and elastin which gives skin its elastic properties. Because of this, they are also responsible for the premature ageing of the skin and the early appearance of wrinkles.
Finally, they can also trigger allergies.
These ultraviolet rays burn the skin. They cause sunburn and erythema or heat rash. Also, there are some which stimulate the synthesis of melanin, the skin’s protective pigment. The UVB rays are therefore responsible for “true” tanning which last several days or even weeks.
When the melanin does not disappear evenly, marks or beauty spots can appear.
In parallel, UVB rays alter the DNA of cells. The human body is indeed capable of responding to the action of ultraviolet rays by a system of DNA repair, but this system cannot cope with prolonged or repeated exposure. It therefore ends up by becoming saturated, leading to the abnormal growth of certain cells and the formation of lesions, which can evolve into forms of skin cancer.
UVC rays are extremely harmful and their carcinogenic power is huge. Fortunately, they are mostly filtered out by the ozone layer. As for the remainder which nonetheless that reaches the earth’s surface, it is stopped by the epidermis.