Sunlight emits visible and invisible wavelengths between 280nm, and 3000 nm. Said radiation is absorbed in part by clouds, the ozone layer, atmospheric contamination and other elements.

The main cause of immunosuppression, premature skin ageing, the formation of free radicals, photo-toxic and photo-allergic reactions and lesions caused by UVA radiation are due to the penetration of these into the connective tissue of the skin.

UVB rays penetrate the deepest cellular layers of the epidermis and are the principle cause of sunburn, DNA-damage and the appearance of skin cancer. Warming IR (infrared) rays and visible light do not damage the skin.

Harmful short-wave UVC radiation should not worry us, since all scientist agree that it never reaches the surface of the earth, because direct and indirect solar radiation wavelengths which constantly bombard the earth, are scattered through the atmosphere, these being the most scattered.

Therefore UVA and UVB radiations are the cause of skin lesion and these also stimulate the photo-protective mechanisms of the organism, in the different skin layers.
Short and controlled exposure to sunlight is sufficient to produce positive effects in the organism.

UV radiation on the skin: effects.

The general principle is that radiation penetrates more and more deeply the longer the wavelength is. Example: UVB radiation at 300nm. penetrates the basal layer of the skin in a high percentage.

UVB radiation is very erythematogenic; the consequences are that sunburn (solar erythema) and later DNA damage trigger off epidermal alterations, acute and chronic lesions, damaging the Langerhans` cells which react to UV radiation by leaving the epidermis. Immunological studies carried out on people submitted to amplified UVB irradiation, have demonstrated these effects without any doubt.

Geographical factors such as latitude and altitude (costal and alpine regions, etc.) as well as the time of day and levels of atmospheric pollution, are the cause of how much UVB radiation reaches the skin. Specifically the reduction of the ozone layer has represented an increase of UVB radiation which reaches the earth and skin in solar exposure.

Irreversible lesions in the long term as well as the premature ageing of the skin and the formation of free radicals are produced by the deep penetration of the connective tissue by the UVa rays, moreover:

Photo-allergic and photo-toxic reactions
These may appear, among other causes, as a reaction to solar exposure of certain ingredients contained in cosmetics, perfumes and by the use of certain medicines.

Formation of oxidants and solar allergies
UV radiation, especially UVA, triggers off cutaneous lesions through the formation of free radicals which damage the cells of the skin. The accumulation through the years of solar exposure may produce chronic lesions. Also they are considered responsible for polymorphic light eruption (PLE) which appears especially in young women.

SUMMARY: UVB and UVA radiation are responsible for cutaneous damage:
UVB radiation due to its deep penetration of the epidermis causes solar erythema, DNA damage and the possibility of skin cancer. UVA radiation due to its penetration of the connective tissue can cause chronic lesions. Moreover it can cause the formation of free radicals and photo-allergic and photo-toxic reactions.