PHOTODAMAGE AND DRY SKIN

Over the past 40 years, considerable evidence has been accumulated from a wide range of experimental studies in animals and humans to clearly indicate that ultraviolet radiation (UVR) from sun exposure has multiple profound effects on skin. Both acute and chronic effects are well described. Ultraviolet radiation is responsible for skin cancer, photoaging, and photosensitivity diseases. In addition, profound immunological effects have been identified which account in part for the beneficial effects of UVR in many diseases such as psoriasis, atopic dermatitis, mycosis fungoid, and vitiligo. Ultraviolet light is artificially divided into very short wave UVC (none currently reaches the earth’s surface), UVB (290 to 320 nm), and UVA, which is divided into UVA II (320 to 340 nm) and UVA I (340 to 400 nm). Ultraviolet A makes up approximately 95% of the UVR to which we are exposed. Until relatively recently, the main focus of research had been directed toward UVB and its role in cancer and immune modulation; UVB wavelengths are far more energetic than UVA and clearly are the dominant factor in squamous cell formation and play an important role in basal cell cancer. In the past decade, in vivo studies in human volunteers have shown that repeated low doses of UVA II and I comparable to those obtained during everyday activities can also have profound effects in skin. Table 1 summarizes the work of many investigators and indicates all wavelengths have profound biological effects on all components and cell types in skin.