	Page 71 -- SKIN AND AGING

   Daily routine exposure to the sun - even in very small doses and in more cloudy climates -can lead to long-term effects that increase our apparent age.
   The part of the sunlight that causes most damage to the skin is called ultraviolet radiation (UVR). For convention and completly arbitarly, the UV spectrum has been divided into UVA,UVB and UVC (see diagram).
   Some exposure to ultraviolet B is necessary, since it is essential for vitamin D production in the skin. The amount required is tiny, however, and 15 minutes a day is probably sufficient even in cloudy countries. Occasional exposure to visible sunlight is believed to enhance psychological well-being. For many years the treatment of skin disorders such as psoriasis has relied on
deliberate controlled exposure to solar simulated sunlight and to UVA lamps.
   In spite of these benefits, however, the UV radiation from the sun is the environmental factor that is overall most damaging to the skin. People who have not been over-exposed to the sun for many years will tend to have pale and unmarked skin; even those with significant pigment will look paler than their kin. Sun worshippers will look very different.

Sun tanning and sunburn

Sun tanning is a response to the damaging effects of ultraviolet radiation. In people with pale skins sunlight stimulates the melanocytes to increase melanin pigment production, and also increases the transfer of melanosomes to keratinocytes. This melanocyte response to sunlight results in tanning, and by dramatically increasing melanin production provide an immediate and important defence for the nuclei of the skin cells.
Initially, acutely sun-damaged skin develops a thickened epidermis. This is caused by faster cell renewal, which is part of the immediate defence mechanism of the skin. The epidermis will return to normal provided the skin is not repeatedly over-exposed.
As nearly everyone knows, acute over-exposure to the sun results in sunburn. Intense redness is produced by increased blood flow due to the release of chemicals by damaged cells.