The sun issues group of luminous radiance which attains the surface of the earth: ultraviolet rays (UV), visible and infrared (IR). UV in their group does not show 5 % complete energies, but the organic molecules is very important it and one concluded to it from considerable physiological answers in every field from the living being their impact on. UVA, UVB and UVC are characterised by beaches of length of different waves. The energy of every radiance depends on its luminous source (the sun for example) and of the middle in which it spreads (the atmosphere of the Earth if the source is the sun).

In general, it can be said that the more the length of wave of a radiance is small, the more its relating energy is important and less he enters circles where he spreads.

UVC corresponds to the fraction of UV which is passed on only in air. These are the radiance the length of waves of which is the shortest (190 - 290 nm), they are therefore extremely energy. This energy confers on them a power of considerable impairment on the biological molecules, they are besides used for their germicidal activity in hospitals. One of their targets being the DNA, they have a very important carcinogenic potential. If the totality of UVC generated by the sun attained the surface of the earth, the toxicity which they would procreate such would be as life would be impossible.

Fortunately the fraction of UVC which joins the surface of the earth is trifling, since from their arrival in the atmosphere, there happens with the ozone a photochemical reaction; so the almost totality of the energy of UVC is absorbed and there is liberalisation of oxygen. Finally l insignificant fraction of UVC which succeeds on the skin is stopped by the epidermis (superficial layer of the skin)

UVB corresponds to the traction of UV which is passed on in the quartz but not in the glass. Their beach of length of waves goes from 290 to 320 nm. They represent 2 % parts UV who attains the surface of the earth and they are very energy. They penetrate partly into the skin and l O to 20 % attain the dermis (layer located under the epidermis)

UVB is responsible for the solar erythema (sunstroke) with its maximum of intensity 24 hours after exhibition. This reaction stimulates the process of m lanog n se of the skin at the origin of postponed pigmentation, process allocated so to a great extent to UVA. Their strong energy can cause the necrosis of the cells of the epidermis. These harmed cells have the name of sunburn cells and liberate chemical mediators who contribute to the inflammatory demonstrations of sunstroke.

The important energy of UVB also allows them to generate molecular riot (impairment of some links of the DNA and of proteins) which in the long term, and if they renew themselves, saturates the system of repairing of 1 ADN. It draws "errors" away  in the genome of the attained cells, which are at the origin of cutaneous cancers; it is for it that they speak about direct toxicity for UVB.

UVA crosses the glass. Their length of waves is included between 320 and 400 nm (short UVA or WA 2 between 320 and 340 nm, of long WA or UVA 1 between 340 and 400 nm) is differentiated. Among UV who attains the earth, 98 % are UVA. Less energy than UVB, they have therefore a bigger power of penetration in the skin and 20 - 30 % attain the medium dermis.

UVA is at the origin of the  phenomenon of Meirowski  , visible taupe colouring straight after that a long exhibition and easing progressively in some hours. This demonstration does not seem to have damaging impact on cutaneous physiology and some people think even that she is actively involved in solar protection, on the other hand, it reveals an important exhibition.

UVA also stimulates the synthesis of the m lanine: it is  brouzage UVA  or postponed pigmentation UVA causes cutaneous erythemata hardly, for it of important doses are requested (20 - 50 J / cm2). So the doses of necessary UVA to cause the same erythema as that caused naturally by UVB have to be thousand upper times in those essentials with UVB. They exist only in the artificial state in the cabins of suntan, that is why the time of exhibition in these cabins must be imperatively limited. UVA does not have direct toxicity, but their radiance generates the training of radical free, what their associate a  indirect toxicity  and also allocates them carcinogenic properties.

Why is it also necessary to protect the skin of UVA?

The effects of WB are well enough known currently. The skin displayed to the solar radiation without photo protector contracts fast a sunstroke which draws away an inflammation (erythema) manifesting itself by redness and pain. These erythemata are procreated well by UVB and not by UVA. For this reason, UVA not having short-term visible damaging effects was considered for a long time as inoffensive.

However their action is sly, resulting from cellular canul es attacks in the course of exhibitions. Indeed, although UVB is the main representatives for the cutaneous cancers, UVA also has a contribution in this type of attack. They are at the origin of production of radical free, short-term extremely reactive molecules of life, who begin reactions in corrupting chains 1 ADN, proteins and lipids and who as a result generate cancerous cells. The impact of WA in some tumours pidermiques (carcinoma pilepto de) was apparently established. UVA is also considered to be a factor favouring the risk of m lanomes.

Molecular impairments led by UVA are also at the origin of the premature ageing of the skin and particularly structures of the dermis. The report collagen insolvable soluble/collag ne diminishes so as to make vary the mechanical properties of the skin (elasticity, thickness). When these cutaneous modifications are accompanied with a slimming of the epidermis, of the appearance of wrinkles and pigmentary stains, all these changes are regrouped under the name of lastose solar energy.

When the skin is protected by a photo protector covering principally the spectre UVB (290-320 nm), they support a solar exhibition all the longer as they risk it it of the sun which constituted an alarm signal do not exist any more. In most cases, the more indication of protection UVB is high, the more subject remains displayed for a long time, augmenting the dose of UVA accepted by the skin. It is therefore necessary all the more to protect the skin against UVA where it is very strongly protected against UVB (photo protectors pointing out a coefficient of protection SPF superior in 25 can attain this indication only if they have a photo protection against 1 WA).

Finally, the important role of UVA was shown in the launching of the Lucite Estivale B nigne or LEB, photodermatosis characterised by reactions of allergic type the agent of which was not apparently identified, radiance UVA that can act as shutter release, UVB as potentialisateur. Indeed a photo apposite protection with strong cover UVA allowed to diminish the demonstrations of LEB, but the product also had a strong cover UVB..