For a six billion-year-old star, the sun is certainly in the news a lot lately, mainly because it is still a source of uncertainty and confusion to many of us.
The center of this confusion is the sun's ultraviolet A (long-wave) and ultraviolet B (shortwave) rays. Our understanding of exactly what kinds of damage each causes to the skin, and how best to protect ourselves, seems to shift every year as new research comes out. For example, it was once thought that only UVB was of concern, but we keep learning more and more about the damage caused by UVA. And new, improved forms of protection against UVA keep emerging. Keeping up with these new developments is a worthwhile challenge that can help all of us prevent sun damage.
What is Ultraviolet Radiation?
UV radiation is part of the electromagnetic (light) spectrum that reaches the earth from the sun. It has wavelengths shorter than visible light, making it invisible to the naked eye. These wavelengths are classified as UVA, UVB, or UVC, with UVA the longest of the three at 320-400 nanometers (nm, or billionths of a meter). UVA is further divided into two wave ranges, UVA I, which measures 340-400 nanometers (nm, or billionths of a meter), and UVA II which extends from 320-340 nanometers. UVB ranges from 290 to 320 nm. With even shorter rays, most UVC is absorbed by the ozone layer and does not reach the earth.
Both UVA and UVB, however, penetrate the atmosphere and play an important role in conditions such as premature skin aging, eye damage (including cataracts), and skin cancers. They also suppress the immune system, reducing your ability to fight off these and other maladies.
UV Radiation and Skin Cancer
By damaging the skin's cellular DNA, excessive UV radiation produces genetic mutations that can lead to skin cancer. Both the U.S. Department of Health and Human Services and the World Health Organization have identified UV as a proven human carcinogen. UV radiation is considered the main cause of nonmelanoma skin cancers (NMSC), including basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). These cancers strike more than a million and more than 250,000 Americans, respectively, each year. Many experts believe that, especially for fair-skinned people, UV radiation also frequently plays a key role in melanoma, the deadliest form of skin cancer, which kills more than 8,000 Americans each year.
Most of us are exposed to large amounts of UVA throughout our lifetime. UVA rays account for up to 95 percent of the UV radiation reaching the Earth's surface. Although they are less intense than UVB, UVA rays are 30 to 50 times more prevalent. They are present with relatively equal intensity during all daylight hours throughout the year, and can penetrate clouds and glass.
UVA, which penetrates the skin more deeply than UVB, has long been known to play a major part in skin aging and wrinkling (photoaging), but until recently scientists believed it did not cause significant damage in areas of the epidermis (outermost skin layer) where most skin cancers occur. Studies over the past two decades, however, show that UVA damages skin cells called keratinocytes in the basal layer of the epidermis, where most skin cancers occur. (Basal and squamous cells are types of keratinocytes.) UVA contributes to and may even initiate the development of skin cancers.
UVA is the dominant tanning ray, and we now know that tanning, whether outdoors or in a salon, causes cumulative damage over time. A tan results from injury to the skin's DNA; the skin darkens in an imperfect attempt to prevent further DNA damage. These imperfections, or mutations, can lead to skin cancer.
Tanning booths primarily emit UVA. The high-pressure sunlamps used in tanning salons emit doses of UVA as much as 12 times that of the sun. Not surprisingly, people who use tanning salons are 2.5 times more likely to develop squamous cell carcinoma, and 1.5 times more likely to develop basal cell carcinoma. According to recent research, first exposure to tanning beds in youth increases melanoma risk by 75 percent.
UVB, the chief cause of skin reddening and sunburn, tends to damage the skin's more superficial epidermal layers. It plays a key role in the development of skin cancer and a contributory role in tanning and photoaging. Its intensity varies by season, location, and time of day. The most significant amount of UVB hits the U.S. between 10 AM and 4 PM from April to October. However, UVB rays can burn and damage your skin year-round, especially at high altitudes and on reflective surfaces such as snow or ice, which bounce back up to 80 percent of the rays so that they hit the skin twice. UVB rays do not significantly penetrate glass.
Protect yourself from UV radiation, both indoors and out. Always seek the shade outdoors, especially between 10 AM and 4 PM. And since UVA penetrates glass, consider adding flat, tinted UV-protective film to your car's side and rear windows as well as to house and business windows. This film blocks up to 99.9 percent of UV radiation and lets in up to 80 percent of visible light.
Outdoors, dress to limit UV exposure: Special sun-protective clothes with UPF (ultraviolet protection factor) indicate how much UV radiation can penetrate the fabric; the higher the UPF, the better. A shirt with an UPF of 30, for example, means that just 1/30th of the sun's UV radiation can reach the skin. Laundry additives can also be washed right into regular fabrics to provide higher UPFs. However, you can enhance your sun safety simply by learning to evaluate everyday fabrics' sun protection qualities and choosing those with the best protection. For instance, bright- or dark-colored, lustrous clothes reflect more UV radiation than do pastels and bleached cottons; and tightly woven, loose-fitting clothes provide more of a barrier between your skin and the sun. Finally, broad-brimmed hats and UV-blocking sunglasses help shield the sensitive skin on your head, neck, and around the eyes - areas that usually sustain a lot of sun damage.
More tips on sun safety can be found in our Guidelines.
Sun Protection Factor (SPF) and UV Radiation
Since the advent of modern sunscreens, a sunscreen's efficacy has been measured by its sun protection factor, or SPF. SPF is not an amount of protection per se. Rather, it indicates how long it will take for UVB rays to redden skin when using a sunscreen, compared to how long skin would take to redden without the product. For instance, someone using a sunscreen with an SPF of 15 will take 15 times longer to redden than without the sunscreen. An SPF 15 sunscreen screens 93 percent of the sun's UVB rays; SPF 30 protects against 97 percent; and SPF 50, 98 percent. The Skin Cancer Foundation maintains that SPFs of 15 or higher are necessary for adequate everyday protection. For more extended or intense sun exposure, the Foundation recommends SPFs of 30 or higher.
Since both UVA and UVB are harmful, you need protection from both kinds of rays. To make sure you're getting effective UVA as well as UVB coverage, look for a sunscreen with an SPF of 15 or higher, plus some combination of the following UVA-screening ingredients: stabilized a avobenzone, ecamsule (a.k.a. MexorylTM), oxybenzone, titanium dioxide, and zinc oxide. You may see the phrases multi spectrum, broad spectrum or UVA/UVB protection on sunscreen labels, and these all indicate that some UVA protection is provided. However, because there is no consensus on how much protection these terms indicate, such phrases may not be entirely meaningful.
There are currently 17 active ingredients approved by the FDA for use in sunscreens. These filters fall into two broad categories: chemical and physical. Most UV filters are chemical: They form a thin, protective film on the surface of the skin and absorb the UV radiation before it penetrates the skin. The physical sunscreens are insoluble particles that reflect UV away from the skin. Most sunscreens contain a mixture of chemical and physical active ingredients.
|Active Ingredient/UV Filter Name||Range Covered|
|UVA1: 340-400 nm|
|UVA2: 320-340 nm|
|UVB: 290-320 nm|
|Aminobenzoic acid (PABA)||UVB|
|Ecamsule (Mexoryl SX)||UVA2|
|Ensulizole (Phenylbenzimiazole Sulfonic Acid)||UVB|
|Meradimate (Menthyl Anthranilate)||UVA2|
|Octinoxate (Octyl Methoxycinnamate)||UVB|
|Octisalate ( Octyl Salicylate)||UVB|
|Titanium Dioxide||UVB, UVA2|
|Zinc Oxide||UVB,UVA2, UVA1|
If you follow our comprehensive Prevention Guidelines, you can enjoy yourself outdoors while staying protected from both UVA and UVB year-round, whatever the weather, wherever your locale.
- Seek the shade, especially between 10 AM and 4 PM.
- Do not burn.
- Avoid tanning and UV tanning booths.
- Cover up with clothing, including a broad-brimmed hat and UV-blocking sunglasses.
- Usa a broad spectrum (UVA/UVB) sunscreen with an SPF of 15 or higher every day. For extended outdoor activity, use a water-resistant, broad spectrum (UVA/UVB) sunscreen with an SPF of 30 or higher.
- Apply 1 ounce (2 tablespoons) of sunscreen to your entire body 30 minutes before going outside. Reapply every two hours, or immediately after swimming or excessive sweating.
- Keep newborns out of the sun. Sunscreens should be used on babies over the age of six months.
- Examine your skin head-to-toe every month.
- See your physician every year for a professional skin exam.
Published on May 24, 2013. Updated on September 20, 2017.
John H. Epstein, MD
Stephen Q. Wang, MD