The skin is the largest human organ. It represents the border between the inner and the outer body and protects it from the negative influences of the environment. One of the major tasks of the skin is to protect the body from the deleterious effects of sunlight.

Our illustration shows the spectrum of the sunlight and the influence of the sunrays on the various skin (epidermis, dermis) layers.

Solar irradiation is an electromagnetic radiation which may be divided into ultraviolet (UV) irradiation, visible light and warming infrared (IR) irradiation. In terms of UV irradiation, we distinguish between short-waved UVB irradiation (290-320 nm), long-waved UVA-irradiation (230-400 nm) and very short-waved UVC irradiation (100-290 nm). UVC is absorbed by the ozone belt of the earth's atmosphere and does not hit the earth's surface.

UVB irradiation mainly leads to sunburn, an inflammation of the skin which leads to a painful swelling and reddening of the skin. UVB irradiation is the main factor with regard to the risk of skin cancerogenesis, however, UVA irradiation also plays a role here.

Primarily, UVA irradiation is held responsible for extrinsic skin aging. Due to research results it is assumed that the immune system is negatively influenced if skin is exposed to large doses of UVA and UVB irradiation. It is also assumed that UV rays not only lead to mutations in skin cells but that they also suppress the surveilling function of the skin's immune system, thus promoting the formation of skin cancer.

According to state-of-the-art knowledge, UVB irradiation is mainly responsible for DNA damage, while UVA irradiation is essentially the originating factor for oxidative stress in the skin.

Infrared (IR) rays of the sunlight are responsible for photobiological mechanisms of skin damage and contribute a further aspect which has been not been sufficiently considered up to now. Scientists assume that part of the IR radiation penetrates into the skin, i.e. epidermis and dermis, and leads to biological effects there. This may result in accelerated skin aging and perhaps in the development of cancer. In cell culture tests it was observed that a reaction is triggered in human skin cells after IR irradiation, leading to the formation of free radicals. These reactive oxygen species result in oxidative stress and has led researchers to develop sun protection agents covering the entire range of UV rays and to incorporate antioxidants counteracting oxidative stress which is caused by IR irradiation.

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