PROTECTING SKIN FROM THE SUN:

AN UPDATE ON SUNSCREENS 

For outdoor enthusiasts concerned about protecting skin from the sun, sunscreen always makes it onto the list of things to bring. Sunscreens protect from ultraviolet rays that are known to produce undesirable skin damage such as sunburn, photoaging, and cancer. But how effectively does sunscreen protect skin? With so many different sunscreens available, how can a choice be made? What do terms such as SPF, broad-spectrum, and water-resistant mean? Is sunscreen sufficient for protection from the sun, or should other methods of protection be employed? This update reviews the role of ultraviolet radiation in skin damage and then addresses various aspects of sunscreens that are important to the consumer.

WHAT IS ULTRAVIOLET?

Ultraviolet (UV) radiation is a component of sunlight characterized by invisible light waves that are shorter and more energetic than the wavelengths of visible light. UV radiation is subdivided into three categories based on wavelength: UV-C (290-100 nm), UV-B (320-290 nm), and UV-A (400-320 nm). UV-A can be further subdivided into UV-AI (400-340 nm) and UV-AII (340-320 nm). Shorter wavelengths of light are more energetic and potentially more destructive than longer wavelengths. Fortunately, UV-C, the shortest wavelengths in the UV spectrum, is completely absorbed by the gases in the atmosphere and does not reach the skin. UV-B and UV-A are of much greater concern.

UV and Skin Damage

            UV-B is primarily responsible for causing skin erythema, more commonly known as sunburn. While UV-B also produces the beneficial effect of Vitamin D synthesis, it is also responsible for skin damage such as pigment changes, keratinocyte hyperplasia, immunosuppression, photoaging, and cancer.²

UV-A also causes sunburn, but to a much lesser degree than UV-B, and many of the potential damaging effects of UV-A were overlooked until early in the last decade. UV-A is now known to also play a part in photoaging and cancer. The potential damaging effects of UV-A warrant concern because UV-A, when compared to UV-B, is approximately twenty times more abundant, is not filtered by window glass, is relatively unaffected by altitude and atmospheric conditions, is relatively constant throughout the seasons, and is 100 times more likely to penetrate into the dermis.  

            The number of skin cancers in the United States is rising, and UV exposure is considered the primary cause for 90 percent of nonmelanoma skin cancer (squamous cell carcinoma and basal cell carcinoma) and 65 percent of malignant melanomas. More than one million nonmelanoma skin cancers are diagnosed annually, and an estimated 59,580 malignant melanomas, the most deadly form of skin cancer, will be identified in 2005.⁶

            UV radiation, primarily UV-B, causes nonmelanoma skin cancers by directly damaging cellular DNA, and also by suppressing the skin’s immune system. UV-B damages DNA by causing base changes in the DNA sequence and inducing the formation of pyrimidine (primarily thymine) dimers.  If this damage occurs in a segment of DNA that codes for p53, an important tumor suppressor gene, the cell loses a vital component of its proofreading mechanism that normally would induce mutated, precancerous cells to self-destruct. Coupled with UV-induced suppression of the skin immune system, another vital component of tumor rejection, these cellular changes increase the probability of developing nonmelanoma skin cancer.

            Sun exposure is also a risk factor for melanoma, although the mechanisms are not as clear. In mouse models, UV-B has been found to be the initiator of melanoma formation. The risk of melanoma is associated with the intensity of sunlight an individual receives over an entire lifetime.

                Whereas UV-B causes direct damage to cellular DNA, UV-A causes indirect damage by promoting the generation of reactive oxygen species. In addition to being a potential cause of skin cancer, UV-A has increasingly been recognized as an important cause of photoaging—a type of skin damage characterized by fine and coarse wrinkles, mottled pigmentation, and roughness. UV-A, particularly UV-AI, causes these photoaging effects by inducing epidermal hyperplasia and destroying the elastic tissue network of skin. Interestingly, in these studies microscopic skin damage was found after exposure to minimal doses of UV-AI, even in the absence of sunburn. These findings have important implications for the use of sunscreens as sun protection.

SUNSCREENS AS SUN PROTECTION

Sunscreens reduce the formation of thymine dimers, which reduce the risk of developing skin cancer and other types of skin damage.  Routine sunscreen use reduces solar elastosis, actinic keratoses, and squamous cell carcinomas.

            In 1999 the US Food and Drug Administration published a monograph for sunscreens, which enables manufacturers to market sunscreens as over-the-counter products.  This monograph covers aspects of sunscreen production and marketing such as acceptable ingredients, doses, formulation, and labeling. The sixteen approved active sunscreen ingredients, their respective maximum allowable concentrations, and their ranges of UV protection are listed in Table 1.

Sunscreen ingredients can be categorized into chemical absorbers or physical blockers based on their mechanisms of action. Chemical absorbers work by absorbing UV radiation and converting its potentially harmful energy into a lower energy form that is dispersed harmlessly. Physical blockers work primarily by reflecting, scattering, and blocking UV radiation.

Of the sixteen sunscreen ingredients listed in Table 1, nine do not offer any significant protection against UV-A, probably because, until recently, UV-B was viewed as the primary mediator of skin damage. Armed with current knowledge about the potential damaging effects of UV-A, sunscreen manufacturers now combine ingredients to create a formulation that protects against both UV-B and UV-A. Formulations of common sunscreens currently on the market can be found at the Environmental Working Group and Healthtouch websites. Of the sixteen FDA-approved sunscreen ingredients, only two offer significant protection against UV-AI, the range of UV-A responsible for photoaging of the skin.  Avobenzone (Parsol^® 1789), with an absorbance spectrum of 310-400 nm, is the only chemical absorber approved in the United States that protects against the full UV-AI spectrum. Tetraphthalydine dicamphor sulfonic acid (Mexoryl^® SX) also offers full UV-AI protection, and is available in Europe.  Zinc oxide, one of the two physical blockers, is the only approved sunscreen ingredient—absorbers and blockers—that offers protection against UV-B, UV-AII, and UV-AI. However, zinc oxide only covers the UV-AI spectrum up to 380 nm. In general, physical blockers offer a broader spectrum of protection than chemical absorbers, but have the undesirable quality of having a dense white appearance when applied. Of the two physical blockers, zinc oxide has been found to be superior to titanium dioxide because it has a broader spectrum of protection and in leaving less residue.

SPF            Sun Protection Factor (SPF) is a ratio that reflects a sunscreen’s ability to prevent sunburn.
Because UV-B is the form of UV primarily responsible for causing sunburn, the SPF value is primarily a measure of UV-B protection. Currently the SPF value indicates little about UV-A protection. Two sunscreens with the same SPF value, but composed of different ingredients can offer significantly different protection against photoaging.
Most consumers understand correctly that sunscreens with higher SPF values prolong the sun exposure time required to develop sunburn. However, because SPF values do not reflect protection from damage by UV-A and other unknowns of sun exposure, sunscreens should not be used to prolong time in the sun. Unfortunately, sunscreens with higher SPF values often lead individuals to spend more time in the sun, which increases an individual’s cumulative sun exposure and risk of developing skin damage.

            Sunscreens rated SPF 15 filter 93 percent of UV-B, which is adequate protection from sunburn for most. Sunscreens rated SPF 30 filter 97 percent of UV-B. Although this additional 4 percent may seem minimal, subjects who applied sunscreen rated SPF 15 had more microscopic skin damage even without visible signs of sunburn than those who applied sunscreen rated SPF 30.²

            The average consumer does not receive the full sun protection indicated by the SPF value. Although the SPF value is determined in the laboratory with a sunscreen layer of 2 mg/cm² thickness, the average research subject applies a layer of sunscreen of 0.5 to 1 mg/cm² thickness, or less.^28-30

SUNSCREEN PRODUCT LABELS

            To decrease consumer misunderstanding on the proper uses of sunscreen, the FDA included regulations in the monograph that defined the terms allowed on sunscreen product labels.¹⁹ Sunscreens with SPF values of greater than 30 must be labeled SPF 30+ because testing methods for products with higher SPF values are inadequate, and because higher SPF values may lead consumers to spend more time in the sun. Misleading or ambiguous statements such as “All Day Protection” and the number of hours of sun protection are prohibited.

The label of “Broad-Spectrum” or “Full-Spectrum” requires that the sunscreen provide UV-B and at least UV-AII protection. The label “Water-Resistant” requires that the product maintain its SPF level after 40 minutes of water immersion, and the term “Very Water-Resistant” (formerly “Waterproof”) requires that the product maintain its SPF level after 80 minutes of water immersion.

For the time being, however, many sunscreen containers are not in accord with these labeling rules. Even though these rules were initially intended to take effect in 2001, the FDA has put a hold on their implementation until further issues concerning UV-A protection are worked out.³¹

UV AND SPECIAL OUTDOOR CONDITIONS

            Certain outdoor conditions may modify UV radiation exposure. Cloudy days may lead individuals to think that they do not need to be as aggressive in practicing sun protection, but even on overcast days up to 90 percent of UV radiation can penetrate the cloud cover. Depending on the type of cover, an individual in the shade can still be exposed to up to 50 percent of UV radiation. Snow and ice reflect up to 80 percent, sand reflects up to 25 percent, and grass and water reflect up to 5 percent of UV radiation.³² UV radiation can penetrate up to 60 cm of water, so swimmers receive little protection while in the water.² UV exposure also increases 8 to10 percent with each 1000-foot gain in elevation.³³ UV radiation exposure is also dependent on latitude—in the summer, UV radiation is two to three times more intense at the equator than in northern Europe.³⁴

RECOMMENDATIONS FOR SUN PROTECTION

            The bottom line is that sunscreens, while important, are inadequate as the sole form of sun protection. Following are some recommendations for comprehensive sun protection regimen.

n      Staying Out of the Sun During Peak Exposure Hours. Levels of UV radiation, particularly UV-B, are highest from 10 am to 2 pm. A general rule is to minimize time in the sun when a person’s shadow is shorter than his height, particularly during the summer.

n      Covering the Skin with Clothing. All fabrics serve as physical barriers to the sun, but some are much more effective than others. Most clothing worn in summer months has an SPF in the range of 6 or 7. Specially manufactured sun-protective clothing has an SPF of 30 or higher. In general, sun protective clothing is tightly woven or has a coating. (Color plays a minor role, but in general darker colors protect better than lighter colors.) Garments made from this material tend to be hot because the fabric blocks the passage of cooling air even with vents in areas not exposed to direct sunlight. Two manufacturers that specialize in sun protective clothing have websites at  www.sunprecautions.com and www.coolibar.com/. However, similar sun protective clothing is available from other sources. (The protection provided by most sun protecting clothing decreases when it is wet.)

n      Wearing a Wide-Brimmed Hat. Because the face, ears, neck, and scalp are the areas of the body most often exposed to the sun, a hat is an essential component of the sun protection regimen. A hat with a four-inch circumferential brim is required to cover the entire face and neck.

n      Applying Generous Amounts of a Broad Spectrum, High SPF Sunscreen. The sunscreen should contain among its list of active ingredients either avobenzone (Parsol^® 1789) or zinc oxide. In order to obtain the full SPF level of protection as indicated on the label, approximately one ounce—one quarter of a four-ounce bottle—should be applied to the entire 1.5 m² surface area of the body. Particular attention should be paid to commonly neglected areas, such as the back of the neck, under the chin, ears, and any areas of the scalp with thin hair. Sunscreen should be applied half an hour before sun exposure to allow it to penetrate and bind to the skin. Reapplications should be made every few hours—more frequently if activities include swimming or excessive sweating. In recent years spray-on sunscreens have grown in popularity, but these have the potential for leaving some areas of skin unintentionally exposed.
In conclusion, sunscreens are an essential component of the sun protection regimen, particularly for outdoor enthusiasts who spend long hours in the sun. But sunscreen alone is not sufficient and should be combined with other methods of sun protection. The FDA says it well in their “Sun Alert” statement, which every bottle of sunscreen will contain when the FDA sunscreen labeling rules take effect: “Limiting sun exposure, wearing protective clothing, and using sunscreens may reduce the risks of skin aging, skin cancer, and harmful effects of the sun.”