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Pronunciations deoxyribonucleic acid fluorouracil melanin melanocytes melanoma ribonucleic acid squamous tretinoin

The skin shields the rest of the body from the sun's rays.

Ultraviolet Light: Ultraviolet (UV) light, although invisible to the human eye, is the component of sunlight that has the most effect on skin. UV light is classified into three types, ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC), depending on its wavelength.

UV light in small amounts is beneficial because it helps the body produce vitamin D Some Trade Names
See Ergocalciferol
. However, larger amounts of UV light damage deoxyribonucleic acid (DNA—the body's genetic material) and alter the amounts and kinds of chemicals that the skin cells make. These changes are responsible for the damaging effects of UV light, including burning, premature skin aging, wrinkling, and skin cancer. Although UVA penetrates deeper into the skin, UVB is responsible for more of the damaging effects of UV light.

The amount of UV light reaching the earth's surface is increasing, especially in the northern latitudes. This increase is attributable to depletion of the protective ozone layer high in the atmosphere. Ozone, a naturally occurring chemical , blocks much UV light from reaching the surface of the earth. Chemical reactions between ozone and chlorofluorocarbons (chemicals in refrigerants and spray can propellants) are depleting the amount of ozone in the protective ozone layer. The amount of UV light reaching the earth's surface also varies depending on other factors. UV light is more intense between 10 am and 3 pm, in the summer, and at higher altitudes. Smoke and smog filter out much UV light, but UV rays may pass through light clouds, fog, and about 1 foot of clear water.

Natural Protection: The skin undergoes certain changes when exposed to UV light to protect against damage. The epidermis (the skin's uppermost layer) thickens, blocking UV light. The melanocytes (pigment-producing skin cells) make increased amounts of melanin, which darkens the skin, resulting in a tan. Melanin absorbs the energy of UV light and helps prevent the light from damaging skin cells and penetrating deeper into the tissues.

Sensitivity to sunlight varies according to the amount of melanin in the skin. Darker-skinned people have more melanin and therefore greater protection against the sun's harmful effects, although they are still vulnerable to some extent. The amount of melanin present in a person's skin depends on heredity as well as on the amount of recent sun exposure. Some people are able to produce large amounts of melanin in response to UV light, whereas others produce very little. People with albinism (see Pigment Disorders: Albinism) are born being able to make little or no melanin at all.

Sunlight and Skin Damage: Exposure to sunlight prematurely ages the skin. Exposure to UV light is responsible for the wrinkles, both fine and coarse; irregular pigmentation; redness; and leathery, rough texture of sun-exposed skin. Although fair-skinned people are most vulnerable, anyone's skin will change with enough exposure.

The more sun exposure people have, the higher their risk of skin cancers, including squamous cell carcinoma, basal cell carcinoma, and malignant melanoma (see Skin Cancers: Melanoma).

Treatment: The key to minimizing the damaging effects of the sun is avoiding further sun exposure. Damage that is already done is difficult to reverse. Moisturizing creams and makeup help hide wrinkles. Chemical peels, alpha-hydroxy acids, tretinoin Some Trade Names
AVITARENOVARETIN-A
creams, and laser skin resurfacing may improve the appearance of thin wrinkles and irregular pigmentation. Deep wrinkles and substantial skin damage, however, require significant treatment to be reversed.

Actinic Keratoses: Precancerous Growths Actinic keratoses (solar keratoses) are precancerous growths caused by long-term sun exposure. These growths are usually pink or red and appear as flaky, scaly areas. They may also be light gray or brown and feel hard, rough, or gritty. The surrounding skin often appears thin. Actinic keratoses usually can be removed by freezing them with liquid nitrogen (cryotherapy); however, if a person has too many growths, a liquid or cream containing fluorouracil Some Trade Names CARAC may be applied. Often, during such treatment, the skin temporarily looks worse because fluorouracil Some Trade Names CARAC causes redness, scaling, and burning of the keratoses and of the surrounding sun-damaged skin. A relatively new drug, imiquimod Some Trade Names ALDARA , is useful in treating actinic keratoses because it helps the immune system to recognize and destroy cancerous skin growths.

Last full review/revision August 2007 by Robert J. MacNeal, MD