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It is through ultraviolet (UV) rays that the sun acts in a positive way upon our body, by participating in the synthesis of Vitamin D. But in order for this synthesis to occur, we only need 10 minutes of sun per day. Beyond this time, if you expose yourself to it carelessly you risk sun damage and allergies in the short term and some ageing in the long term!
Ultraviolet rays are at the origins of tanning as they stimulate the production of melanin, the molecule which pigments the skin. We distinguish between the UVBs, directly responsible for sun damage and sunburn, and UVA which acts in a deeper way and over the long term. UVC rays also exist: these ultraviolet rays are extremely carcinogenic but they get mentioned last because most of them are filtered out by the ozone layer and those that reach the ground are stopped by the epidermis.
By reacting to the pigments already present in the skin, UVA rays make us tan, but to a shade which does not last. When they reach the dermis, they lead to the production of free radicals and alter the membrane of skin cells but also their DNA. So, beyond the risk of cancers, they are responsible for premature skin ageing. They also act upon the molecules of collagen and elastin that gives the skin its elastic properties. A as in Age…
Finally, the UVA rays are also at the start of allergies to the sun that some people develop.
UVB rays activates the synthesis of melanin, the protective pigment of the skin, and are therefore responsible for ‘true’ tanning, that which lasts several days or even several weeks. But these rays can also cause sun damage and erythema. B as in Bronze-hued tan, but also as in Burns...
A different impact according to the type of skin
In the long term, the appearance of sun spots affects all types of skin, but not at the same time. Indeed, our genetic background makes us react in a different way. Light skins are less well armed to defend themselves against U rays than dark skins.