The character and amount of such radiation vary greatly with the seasons and with changing atmospheric conditions. Exposure of skin to sunlight depends on multiple factors, eg, clothing, lifestyle, occupation, and geographic factors, such as altitude and latitude. Snow, sand, and bright sky enhance exposure by reflecting the rays. Even light sources that contain only UVA adversely affect the skin. After exposure to sunlight, the epidermis thickens and melanocytes produce melanin at an increased rate, providing some natural protection against future exposure.
Persons differ greatly in their reactivity to sunlight.
Pigmentation does not occur in the skin of albinos because of a defect in melanin metabolism, nor does it occur in areas of vitiligo because of the absence of melanocytes. Blacks and other nonwhites are less sensitive to sun exposure than fair-skinned persons, but they are not immune to the effects of the sun and can become sunburned with prolonged exposure. Persons with blonde or red hair are especially susceptible. Uneven melanin deposition occurs in many fair-haired persons and results in freckling. All rights reserved.