Even if we perceive sun rays as being homogeneous and of a unique type, they are actually divided in three categories that have different effects upon our body: UVA, UVB and UVC rays. Therefore, the ultraviolet (UV) radiation is of A, B and C type.
Out of these, only UVA and UVB types are harmful to our skin, damaging its surface and inner structure when taking prolonged sunbaths. This happens because the third type, the UVC radiation, does not penetrate the atmosphere's ozone layer and does not reach Earth.
However, UVB and UVA act differently upon our skin and do not equally damage it. The UVB rays are mostly responsible for most cases of sunburn, as they are shorter than UVA rays and only reach the surface of the skin that is made up of the epidermis layer. The UVB radiation is absorbed by the stratum corneum on the epidermis surface. This stratum corneum is the dead cells layer - the visible layer of the skin. The UVA radiation is partly absorbed and blocked by the ozone layer before reaching Earth.
The UVA damage is deeper, as the UVA rays are longer than the UVB ones and reach the inner strata of the skin. They are responsible for causing the skin to lose its elasticity and wrinkling, leading to premature aging of the skin. They also can burn the skin, but at a deeper level. The UVA rays are not at all absorbed by the atmosphere and completely reach Earth.
Both UVA and UVB lead to skin cancer, whether it is malignant or benign. In the malignant type of cancer there is a tumor that appears as a consequence of abnormal proliferating skin cells. The uncontrollable growing of these cells leads to melanoma tumor forming, which in most of the cases is lethal.
Melanoma is a cancerous skin tumor which is produced by the cells in the skin that give its pigment (melanin), cells called melanocytes. Melanoma begins as a dark skin lesion and may spread rapidly to other areas on the skin and within the body. Usually, melanoma skin cancer is caused by the UVA rays, as these are longer and can penetrate deeper skin layers.
The non-melanoma skin cancer is not that dangerous for our lives, but it is nevertheless slowly growing within our epidermis. The most likely to get skin cancer are the fair-skinned people that also get burned when over-exposing to the sun. The skin cancer which does not involve a tumor is usually cause by the UVB radiation that is limited to the surface of the skin.
UV rays can also cause other disorders of the skin besides cancer, such as photoaging, actinic keratoses, lupus vulgaris (tuberculosis of the skin), psoriasis or vitiligo (a discontinuous depigmentation of the skin.)
Besides skin disorders and cancer, there is another wide range of conditions, mild or severe, caused by UV radiation to humans. This includes eye disorders - photokeratitis (inflammation of the cornea), photoconjunctivitis (inflammation of the conjunctiva), pterygium (growth of the conjunctiva on the surface of the eye), cataracts (the main cause for blindness) etc. UVA are responsible for most of the vision problems, as they do not limit their action to the surface of the eye, but they go deeper, penetrating the retina.
Excess sunbathing may also lead to weakening our immune system. This happens because UV radiation can alter the distribution and function of white blood cells in humans that are known for their active fighting-diseases role, for up to 24 hours after sun exposure. Repeated and excessive sunbaths can cause higher damage to the immune system.
Therefore, everyone should protect himself or herself from the sun. We all know that sun has also beneficial effects on our health, but this can be obtained only by scheduling our sunbaths early in the morning and also in the evening and avoiding midday. We should also pay extremely high attention to always use radiation protective sunscreens, sunglasses, hats etc.
And as beautiful as a tanned skin can be and look, do not forget that the tan is actually the way in which skin reacts when is being injured. Tanning occurs when the UV rays penetrate the skin's base layer and makes the skin to produce more melanin (skin's natural pigment) as a response to the injury.