Actinic keratoses (AKs) are lesions caused by intense and chronic exposure to sunlight—especially the ultraviolet wavelengths of sunlight. AKs are flat, round or irregularly-shaped scaly lesions; surrounding skin is frequently wrinkled, mottled and rough. Other names for AKs include solar keratoses, solar precancerosis, solar cheilitis (when scaling lesions are on the lip) and senile keratosis (AKs most often occur in people over age 40, and visibly contributes to an "aging" appearance).

The best way to prevent AKs is to protect yourself against the skin-damaging effects of sunlight—in particular, the ultraviolet rays of sunlight. This is called photoprotection.

Principles of Photoprotection
Photoprotection is the use of physical or chemical agents to prevent the skin-damaging effects of ultraviolet (UV) radiation in sunlight. Physical photoprotection outdoors is provided by clothing, umbrellas or parasols, awnings, trees or tents. Chemical photoprotection usually means sunscreens applied to the skin—topical sunscreens.

You can also protect yourself by staying out of the sun during peak sunlight hours between 10 a.m. and 4 p.m. Avoiding intense, chronic exposure to solar radiation is a primary means of protection against AKs and premature aging of the skin. This is especially true for people living in areas of year-round high-intensity sunlight (e.g., Florida, Arizona), and for those who spend many hours outdoors in summer months.

However, risk for AKs isn’t limited to sunny climates. A recent study shows that risk for AKs can be substantial even in far northern 
latitudes, especially when a person has AK risk factors of white skin, 
red or blond hair, freckles and blue, green or gray eyes. The investigators studied a light-skinned population of 968 men and women over 40 years of age in northwest England—at about the same latitude as Denmark. The overall prevalence of AKs at all ages was 15.4% in men and 5.9% in women, but in those over age 70 the prevalence was 34.1% in men and 18.2% in women. In those over age 70 with AKs, the presence of AKs was most strongly associated with (1) red hair and freckles, and (2) other indications of photodamage such as skin mottling and loss of skin elasticity. [Reference: Memon AA et al. Prevalence of solar damage and actinic keratoses in a Merseyside population. Br J Dermatol 2000; 142:1154-1159].

Sunscreens for Photoprotection

There are of two basic types of sunscreens:

• Absorbers of UV radiation—chemical sunscreen ingredients such as para-amino benzoic acid (PABA), cinnamates, salicylates and benzophenones
• Reflectors of UV radiation—physical sunscreen ingredients such as titanium oxide or zinc oxide

A third class of topical agents is the antioxidants such as vitamins E and C that do not absorb or reflect UV radiation but are believed to enhance the ability of skin cells to repair damage caused by UV.

Sunscreens usually consist of a combination of several photoprotective chemicals. The degree of protection they provide is identified as a sun protective factor (SPF). The American Academy of Dermatology recommends using a sunscreen with an SPF of at least 15, even on cloudy days. A person with pale white skin, red or blond hair, and blue, gray or green eyes are at highest risk for photoaging, AKs and skin cancer. SPF is determined in the United States by a guideline accepted by sunscreen manufacturers and the Food and Drug Administration (FDA). Topical sunscreens are marketed as lotions, creams, gels, ointments and sprays; the type you select is a matter of personal choice.

An effective sunscreen should also protect against both the A and B wavelengths of UV. Both UVA and UVB are present in sunlight and both can induce skin cell damage. The UVB wavelengths are the principal cause of sunburn. UVA can penetrate to deeper layers of skin. Since most chemical ingredients of sunscreens are maximally effective against either UVA or UVB, many sunscreens are a mixture of UVA and UVB-absorbing chemicals, or physical blocking agents such as zinc oxide that block all UV. You can check the label to see if a sunscreen provides both UVA and UVB protection. A sunscreen providing UVB-only protection is effective in preventing sunburn but less effective against the deeper penetrating UVA. You can also check the sunscreen label for ingredients that provide broad-spectrum (UVA-UVB) protection: benzophenones (oxybenzone), cinnamates (octylmethyl cinnamate and cinoxate), sulisobenzone, salicylates, titanium oxide, zinc oxide, and avobenzone.

In addition to SPF, the UV protection of a sunscreen depends on its ability to remain on the skin over several hours that may include physical exercise, sweating and swimming. A dermatologist can recommend a sunscreen for the maximum protection needed by an individual patient.

Here are some simple rules for improving sunscreen protection:

• Apply sunscreen at least 20 minutes before going outdoors

• Reapply sunscreen every 2 hours, and after swimming or sweating heavily. (But reapplying sunscreen doesn’t increase its SPF—it just maintains your level of protection)

• For whole-body coverage, use about 1 ounce of sunscreen (enough to fill a shot glass), and apply it liberally with special attention to your face, ears, nose, arms and legs

• Remember that lips also need protection. Use a lip balm with SPF of 15 or higher

Side effects of sunscreens, if they occur, are usually a contact irritant reaction to a chemical in the sunscreen. Other side effects that can occur include phototoxicity or photoallergy (due to interaction of a chemical in the sunscreen with sunlight), and contact allergy. A sunscreen that causes a side effect should be avoided. Persons with existing skin conditions such as acne, eczema or other dermatitis, AKs or rosacea should consult a dermatologist regarding selection of an appropriate sunscreen.

What About ‘Natural’ Antioxidant Agents?
Many people today are interested in using "natural" products, whether foods or cosmetics. In this class of sun protective agents are the antioxidants that may act after UV skin penetration to help skin cells repair cellular and DNA damage. Topical antioxidant sun protective agents include:

• Vitamins C and E, both natural antioxidants
• Silymarin (milk thistle extract), an antioxidant of the flavanoid class of molecules present in many foods
• Extracts of the Polypodium plant
• Vitamins C and E in topical form are a treatment for photoaging

• SPF of 15 or higher
• Both UVA and UVB protection
• Does not cause skin irritation
• Does not worsen any existing skin conditions

Selecting Clothing for Photoprotection
For maximum photoprotection, you can select clothing as well as sunscreen on the basis of SPF. Clothing with a high SPF can block nearly 98% of UVA and UVB radiation—a degree of protection especially important a person who (1) burns easily and is at high risk for AKs, photoaging, skin cancer and other sun-induced skin conditions, and (2) is chronically and intensely exposed to sunlight while hiking, gardening, farming, fishing and working outdoors. Good sun protection is important for children as well as older people. Skin damage from solar radiation can begin early, and some epidemiologic studies have indicated that heavy sun exposure in childhood is a risk factor for melanoma (the deadliest form of skin cancer) later in life.

For maximum photoprotection from clothing, here are some guidelines to keep in mind:

• The clothing should provide maximum body coverage—long pants, long-sleeved shirt, wide-brimmed hat, and gloves
• Tightly woven synthetic fabrics made from nylon and polyester provide maximum photoprotection. Tightly woven cotton blends are nearly as photoprotective and more comfortable in high heat and humidity
• Fabrics woven specifically to provide UV protection are made by U.S. manufacturers to meet SPF specifications
• A dermatologist can make recommendations regarding selection of sun-protective clothing for the needs of the individual patient

AAD Dermatology Insights, Spring 2000

Pathak MA et al. Sun-protective agents: formulation, effects, and side effects. In: Freedberg IM et al (Eds.) Fitzpatrick.s Dermatology in General Medicine, 5^th Ed. New York: McGraw-Hill; 1999:2742-2763.

Cadet J, Berger M, Douki T, Ravanat JL. Oxidative damage to DNA: formation, measurement, and biological effects. Rev Physiol Biochem Pharmacol 1997; 131:1-87.