April 16, 2007

Sunlight and Skin Damage


The skin shields the rest of the body from the sun's rays. 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. However, larger amounts of UV light damage DNA (the body's genetic material) and alter the amounts and kinds of chemicals that the skin cells make. UV light also may break down folic acid, sometimes resulting in deficiency of that vitamin in fair-skinned people. Although UVA penetrates deeper into the skin, UVB is responsible for at least three quarters of the damaging effects of UV light, including tanning, burning, premature skin aging, wrinkling, and skin cancer.

The amount of UV light reaching the earth's surface is increasing, especially in the northern latitudes. This increase is attributable to chemical reactions between ozone and chlorofluorocarbons (chemicals in refrigerants and spray can propellants) that are depleting the protective ozone layer, creating a thinner atmosphere with some holes. UV light is more intense between 10 a.m. and 3 p.m., in the summer, and at higher altitudes.

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 (the 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 prevents the light from 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 naturally pale 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) have little or no melanin at all.

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

The more sun exposure a person has, the higher the risk of skin cancers, including squamous cell carcinoma, basal cell carcinoma, and, to some degree, malignant melanoma.

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 creams, and laser skin resurfacing may improve the appearance of thin wrinkles and irregular pigmentation. Deep wrinkles and significant skin damage, however, are unlikely to be reversed

Actinic Keratoses: Precancerous Growths
Actinic keratoses (solar keratoses) are precancerous growths caused by long-term sun exposure. These growths appear as flaky, scaly areas that do not heal; they may also be darkened or gray and feel hard. The surrounding skin often appears thin.

Actinic keratoses usually can be removed by freezing them with liquid nitrogen; however, if a person has too many growths, a liquid or cream containing fluorouracil may be applied. 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.

Photosensitivity Reactions
Photosensitivity, sometimes referred to as a sun allergy, is an immune system reaction that is triggered by sunlight. Photosensitivity reactions include solar urticaria, chemical photosensitization, and polymorphous light eruption and are usually characterized by an itchy eruption on patches of sun-exposed skin. People may inherit a tendency to these reactions. Certain diseases, such as systemic lupus erythematosus and some porphyrias, also may cause the skin to break out in response to sunlight.

Solar urticaria are hives (large, itchy red bumps) that develop after only a few minutes of exposure to sunlight. The hives appear within 10 minutes of sun exposure and go away within an hour or two after leaving the sunlight. People with large affected areas often have headaches and feel weak and nauseated.

Chemical photosensitivity is a condition in which people develop redness, inflammation, and sometimes brown or blue discoloration in areas of skin that have been exposed to sunlight for a brief period. This reaction differs from sunburn in that it occurs only after the person has swallowed certain drugs or chemicals or has applied them to the skin. These substances make some people's skin more sensitive to the effects of ultraviolet light. Some people develop hives with itching, which indicates a type of drug allergy that is triggered by sunlight.

Polymorphous light eruption is an unusual reaction to sunlight, the cause of which is not understood. It is one of the most common sun-related skin problems and is more common in women and in people who are not regularly exposed to the sun. The eruption appears as multiple red bumps and irregular red patches appearing on sun-exposed skin. These patches, which are itchy, generally appear between 30 minutes and several hours after sun exposure; however, new patches may develop many hours or several days later. The bumps and patches usually go away within a week. Typically, people with this condition who continue to go out in the sun gradually become less sensitive to the effects of sunlight.

Diagnosis, Prevention, and Treatment

There are no specific tests for photosensitivity reactions. A doctor suspects a photosensitivity reaction when a rash appears only in areas exposed to sunlight. A close review of any diseases, drugs taken by mouth, or substances applied to the skin (such as drugs or cosmetics) may help a doctor pinpoint the cause of the photosensitivity reaction. Doctors may perform tests to rule out diseases, such as systemic lupus erythematosus, that are known to make someone susceptible to such reactions.

A person with sensitivity to sunlight from any cause should wear protective clothes, avoid sunlight as much as possible, and use sunscreens. If possible, any drugs or chemicals that could cause photosensitivity should be discontinued.

People with polymorphous light eruption or lupus photosensitivity sometimes benefit from treatment with hydroxychloroquine or corticosteroids taken by mouth. For certain types of photosensitivity, treatment can consist of phototherapy (exposure to ultraviolet light) with the use of psoralens (drugs that sensitize the skin to the effects of ultraviolet light). This treatment is called PUVA (psoralens plus ultraviolet A. However, people with systemic lupus erythematosus cannot tolerate PUVA therapy.

Sunburn results from a brief (acute) overexposure to ultraviolet (UV) light. The amount of sun exposure required to produce a burn varies with each person's pigmentation and ability to produce more melanin.

Sunburn results in painful reddened skin. Severe sunburn may produce swelling and blisters. Symptoms may begin as soon as 1 hour after exposure and typically reach their peak after 1 day. Some severely sunburned people develop a fever, chills, and weakness and on rare occasions even may go into shock (characterized by very low blood pressure, fainting, and profound weakness). Several days after a sunburn, people with naturally fair skin may have peeling in the burned area, usually accompanied by itching. These peeled areas are even more sensitive to sunburn for several weeks. People who have had severe sunburns when young are at greater risk of skin cancer in later years even if they have not had long-term sun exposure.


The best—and most obvious—way to prevent sun damage is to stay out of strong, direct sunlight. If sun exposure is necessary, the person should get out of the sun quickly at the first sign of tingling or redness. Clothing and ordinary window glass filter out most of the damaging rays. Water is not a good filter: UVA and UVB light can penetrate a foot of clear water. Clouds and fog are also not good filters of UV light; a person can get sunburned on a cloudy or foggy day. Snow, water, and sand reflect sunlight, magnifying the amount of UV light that reaches the skin. People also burn more quickly at high altitudes, where the thin air allows more burning UV light to reach the skin.

Before exposure to strong direct sunlight, a person should apply a sunscreen, an ointment or cream containing chemicals that protect the skin by filtering out UV light. Most sunscreens tend to filter only UVB light, although some newer sunscreens are somewhat effective at filtering UVA light as well.

Sunscreens contain substances, such as para-aminobenzoic acid (PABA) and benzophenone, that absorb UV light. Because PABA does not immediately bind strongly to the skin, sunscreens containing PABA must be applied 30 to 45 minutes before going out in the sun or into the water. PABA may irritate the skin or cause an allergic reaction in some people. Many sunscreens contain both PABA and benzophenone or other chemicals; these combinations provide protection from a broader range of UV light. Many sunscreens claim to be either waterproof or water-resistant, but most of these nonetheless require more frequent application in people who are swimming or sweating.

Other sunscreens, called sunblocks, contain physical barriers such as zinc oxide or titanium dioxide; these thick, white ointments block almost all sunlight from the skin and can be used on small, sensitive areas, such as the nose and lips. Some cosmetics contain zinc oxide or titanium dioxide.

In the United States, sunscreens are rated by their sun protection factor (SPF) number—the higher the SPF number, the greater the protection. Sunscreens rated between 0 and 12 provide minimum protection; those rated between 13 and 29 provide moderate protection; those rated 30 and above provide maximum protection.


Cold tap water compresses can soothe raw, hot areas, as can skin moisturizers without anesthetics or perfumes that might irritate or sensitize the skin. Nonsteroidal anti-inflammatory drugs (NSAIDs) help relieve pain and inflammation. Corticosteroid tablets also may help relieve the inflammation but are used only for the most serious burns. Specific antibiotic burn creams are required only for severe blistering. Most sunburn blisters break on their own: they do not need to be popped and drained unless they are still intact after 3 or 4 days. Sunburned skin rarely becomes infected, but if an infection develops, healing may be delayed. A doctor can determine the severity of an infection and prescribe antibiotics if necessary.

Sunburned skin begins healing by itself within several days, but complete healing may take weeks. After burned skin peels, the newly exposed layers are thin and initially very sensitive to sunlight and must be protected for several weeks.

Are Tans Healthy?
In a word—no. Although a suntan is often considered an emblem of good health and of an active, athletic life, tanning for its own sake has no health benefit and is actually a health hazard. Any exposure to ultraviolet A or ultraviolet B light can alter or damage the skin. Long-term exposure to natural sunlight causes skin damage and increases the risk of skin cancer. Exposure to the artificial sunlight used in tanning salons is harmful as well, even though the UVA lights used in these establishments are somewhat less likely to produce skin cancer. Quite simply, there is no “safe tan.”

Self-tanning, or sunless, lotions do not really tan the skin but, rather, stain it. They therefore provide a safe way to achieve a tanned look without risking dangerous exposure to UV rays. However, because they do not increase melanin production, self-tanning lotions do not offer protection from the sun. Therefore, sunscreens should still be used during exposure to sunlight. Results with the use of self-tanning lotions may vary, depending on a person's skin type, the formulation used, and the manner in which the lotion is applied.

Adapted from: Merck & Co. Inc.