Sunlight stimulates vitamin D production, triggers an immune response which sends protective and repair cells to the skin, and causes a sense of well-being. Yet sunlight can also cause damage to the skin. Damage includes not only a painful sunburn but skin cancers, wrinkling and other changes associated with aging skin, and even allergic reactions and worsening of some skin diseases. 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 (UVA and UVB) damages deoxyribonucleic acid (DNA—the body's genetic material), which can ultimately lead to cancer. UV light also has damaging effects such as premature skin aging and wrinkling. Sunburn can also result from UV light, primarily UVA. There is no safe level of UV light.

The amount of UV light reaching the earth's surface is increasing, especially in the northern latitudes. This increase is caused by 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 high altitudes and low latitudes (such as at the equator). Smoke and smog filter out much UV light, but UV rays may pass through light clouds, fog, and about 1 foot of clear water, potentially causing severe burns.

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. Tanning provides some natural protection against future exposure to UV radiation, because melanin absorbs the energy of UV light and helps prevent the light from damaging skin cells and penetrating deeper into the tissues. However, tanning for the sake of being tan is hazardous to health (see Sunlight and Skin Damage: Are Tans Healthy?).

Sensitivity to sunlight varies according to the amount of melanin in the skin. Darker-skinned people have more melanin and therefore greater built-in protection against the sun's harmful effects. However, darker-skinned people are still vulnerable to sun damage and the long-term effects of exposure to UV light. 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 blonde or red hair are especially susceptible to the short-term and long-term effects of UV radiation. The melanin in their skin can also become distributed unevenly, resulting in freckling. People with vitiligo (see Pigment Disorders: Vitiligo) have patchy areas of skin that have no pigment. People with albinism (see Pigment Disorders: Albinism) have 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: To minimize the damaging effects of the sun, it is particularly important to avoid further sun exposure, wear protective clothing, and apply sunscreens (see Sunlight and Skin Damage: Prevention). Damage that is already done is difficult to reverse. Moisturizing creams temporarily plump up wrinkles, and makeup helps hide wrinkles. Treatments such as chemical peels, alpha-hydroxy acids, tretinoin 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 irregular, 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) or scraping (curetting) and then burning them with an electrical current (electrocautery). However, if a person has too many growths, a liquid or cream containing fluorouracil (a chemotherapy drug applied to the skin) may be used. Often, during such treatment, the skin temporarily looks worse because fluorouracil causes redness, scaling, and burning of the keratoses and of the surrounding sun-damaged skin. A relatively new drug, imiquimod, is useful in treating actinic keratoses because it helps the immune system to recognize and destroy precancerous skin growths. Imiquimod causes redness, scaling, and burning similar to that caused by fluorouracil. Another treatment is photodynamic therapy, in which a chemical preparation is applied to the skin and then artificial light is applied to the growths. This therapy causes a sunburn-like appearance. The main advantage of photodynamic therapy is that it typically reduces the number of days of skin redness, scaling, and irritation.

Last full review/revision December 2012 by Robert J. MacNeal, MD