Sunscreen

Authors : Suraj Venna Melanoma Center Washington Cancer Institute, Felix Kuo FAAD NY 2009-07-20

(Photo: Fraunhofer Lines Diagram spectrum of visible light ( Sun ). Source : NASA)

Introduction

We are constantly exposed to all types of electromagnetic energy, whether from cellular phones, microwaves, televisions, radio waves, or the ultraviolet light from the sun.
The electromagnetic spectrum includes several types of energy forms, including cosmic rays, gamma rays, X-rays, ultraviolet (UV), visible light (Red, Orange, Yellow, Green, Blue, Violet), infrared, and radio waves. The shorter the wavelength, the more energy it contains and potentially the more damaging (mutagenic) it may be to the building blocks of our cells, our DNA. At one end of this electromagnetic spectrum are very short waves, but extremely high energy and very mutagenic (cosmic, gamma, X-ray). Fortunately, these most damaging types of waves don’t reach earth’s surface. We are exposed to visible light, infrared and ultraviolet ray. The ultraviolet spectrum of light includes UVA, UVB, and UVC. Again, the shorter the wavelength, the more mutagenic it is, thus UVC is most dangerous. However, the ozone layer blocks nearly 100% of UVC, a significant portion of UVB, and little to no UVA. Roughly 100 times more UVA reaches the earth’s surface than does UVB, although photon for photon, the energy in 1 UVB is equal in magnitude to the energy in 1000 UVA photons. UVB rays are thought to be primarily responsible for having a role in causing skin cancer. UVA penetrates skin more deeply and causes wrinkles, sun spots, loss of elasticity of the skin, and other aspects of aging skin. There are intrinsic and extrinsic causes of skin aging. The normal changes of our skin as we age are accelerated with unprotected and excessive sun exposure, termed photoaging. UVA rays recently have been shown to also have a role in skin cancer and thus the best sunscreens are “broad-spectrum,” meaning they block harmful UVA and UVB rays.

Are sunblocks and sunscreens the same thing?

Well, nearly. Sunscreens prevent the adverse effects of sunlight to your skin by either physically or chemically blocking the sun. Sunscreens are divided into Physical and Chemical compounds. The term sunblock usually refers to physical sunscreens. Physical sunblocks are opaque substances that block sunlight by reflecting or scattering UV radiation before it reaches your skin. They contain zinc oxide or titanium dioxide, which are broad spectrum, meaning they block UVA and UVB. Older formulations of these compounds were cosmetically unappealing. These days most physical blockers are in a micronized form which allows for better blending onto the skin. Physical blockers cause far fewer skin reactions as compared with chemical sunscreens, and are well tolerated by those with sensitive skin.

Chemical sunscreens are made of several ingredients to cover both UVA and UVB, the types of UV that cause sunburn and skin cancer. This is because no single chemical ingredient covers both UVA and UVB, unlike the physical blockers which do truly have broad spectrum coverage. These chemicals cover a small range of the UV spectrum and therefore various combinations of several compounds are needed to attain broader coverage. Today many sunscreens contain a mixture of chemical and physical blocking agents.

Until recently, UVA blocking compounds such as avobenzone (aka Parsol 1789), was destabilized by sunlight, meaning that nearly 50% of the active ingredient degraded after 1 hour of sun exposure. However UVA photostabilizers have been produced, allowing for prolonged coverage. Diethylhexanaphthalene (2,6 DEHN) is one such compound. The US Food and Drug Administration recently approved a chemical that has been used in Europe for many years, termed ecamsule (Mexoryl). This compound has a peak absorption of 345nm which is in the UVA spectrum (320-400nm) and also covers UVB, so is truly a broad spectrum compound. Since it peaks in the UVA range, it still needs to be combined with other UVB chemical sunscreens or physical blockers. The following is a list of common ingredients in sunscreen and the type of UV blocked.

Sunblocks and coverage within the UV spectrum

 

CHEMICAL	UVB	UVA2	UVA1
Cinnamates	Yes	No	No
Salicylates	Yes	No	No
PABA	Yes	No	No
Padimate-O	Yes	No	No
Benzophenones	Yes	Yes	No
Avobenzone (aka Parsol 1789)	No	Minimal	Yes
Ecamsule (aka Mexoryl-peak 345nm)	Yes	yes	Yes
PHYSICAL
Titanium Dioxide	Yes	Yes	No (micronized)
Zinc Oxide	Yes	Yes	Yes

What is the SPF or sun protection factor and what number should I use?

SPF is an acronym for sun protection factor, which is rating from 2 - 70 and refers to the length of time a sunscreen remains effective on the skin. However, the SPF designation is based only on UVB blocking and doesn’t tell us anything about a particular product’s UVA blocking ability. Currently the FDA is seeking an analogous rating system to determine precise UVA protection.

For example, if you burn after 10 minutes of being exposed to sunlight without using any sunscreen, then theoretically, if you use a sunscreen with an SPF of 15, it would now take you 10 min x15SPF or 150 minutes to burn. The amount of ultraviolet blocked, and the SPF number is non-linear, meaning that SPF30 doesn’t double the blocking capacity of an SPF15. In reality, SPF15 blocks 93% of ultraviolet light and SPF 30 blocks nearly 95-97%. The most important thing for the consumer is to reapply every 2 hours when exposed.

What level of SPF should I be using?

The American Academy of Dermatology (AAD) recommends that a "broad spectrum" sunblock with an SPF of at least 15 be used daily on all sun exposed areas, then reapplied every 2 hours. The higher the SPF, the longer it protects against sun exposure. We recommend an SPF 15 as found in daily moisturizers and at least SPF 30 or greater for prolonged sun exposure.

How often should I be using sunscreen?

We should all use sunscreen daily since its use also protects against photoaging. Many daily facial moisturizers have a built-in SPF of 15. For prolonged outdoor activities, sunscreen with an SPF of 30 or greater should be used. Remember, even on a cloudy day, 80% of ultraviolet (UV) can still reach us. For you skiers, a significant portion of UV, upwards of 80%, can be reflected off the snow back at you! Also, the intensity of UV increases by 4% every 1000 feet of altitude.

Is there a right way and a wrong way to use sunscreen?

Most of us inadequately use sunscreen. Think about it: a large bottle of sunscreen can last the whole summer, but it shouldn’t! If you spend a day at the beach and are reapplying every 2 hours as you should, then you should be half-way through an 8-ounce bottle by the end of the day! An ounce of sunscreen is about 1 shot glass equivalent.

How long does sunscreen remain potent and does it have a shelf life?

Yes it does! Many of us are probably still using a bottle of sunscreen from last summer or even many summers earlier. Since most sunscreens have chemical blockers, these compounds breakdown with time. Therefore, we recommend once you begin using a bottle of sunscreen, discard after 6 months.

Are sunscreens truly water-proof?

A sunblock is considered water-proof if it maintains its SPF at 80 minutes. A sunblock is considered water-resistant if it maintains its SPF at 40 minutes. We recommend water resistant sunblocks for active individuals or those involved in water sports. The bottom line is that people who engage in outdoor activities, and certainly those who swim, should opt for a water-resistant or water-proof sunscreen.

What should I look for when purchasing a sunscreen?

You want a broad spectrum sunscreen, one that covers both the UVA and UVB ranges, with an SPF of 15 for daily use, and at least 30SPF for prolonged sun exposure (30 minutes or greater). The Skin Cancer Foundation has a Seal of Recommendation embossed on the products that meet rigorous standards. For a sunscreen to attain this seal it must meet the following criteria:

• SPF of 15 or more
• Testing of the sunscreen on a specified number of people
• Evaluated for possible adverse skin reactions
• If labeled water proof, it must prove this through testing

Other forms of sun protection: sunglasses and UPF rated protective clothing

Sunglasses that block UV are an important method of preventing damage to the cornea and other eye conditions. Look for sunglasses that block both UVA and UVB.

As of this writing, there has been no standard format to rate UV blocking ability of sunglasses. That said, the $10 sunglasses for sale by street vendors probably do not have significant UV blocking ability. High-end sunglasses that claim to block 100% of UV contain polycarbonate, glass or a type of plastic polymer (allyl diglycol carbonate), also known as CR-39.

Sun protective clothing is another effective method for blocking UV rays. This clothing comes in all styles and colors and uses an Ultraviolet Protection Factor or UPF to rate a particular garment. Clothing with a UPF of 50 means that 1/50^th of UV actually gets through to your skin, so it blocks 98% of UV. The science and history of sun protective clothing has been summarized in a recent journal article (1). The standards for UPF are validated for clothing by several organizations including the American Association of Textile Chemists and Colorists (AATC) and the American Society for Testing and Materials. The Skin Cancer Foundation has bestowed its seal of approval to Coolibar, a company that sells UPF rated clothing that has been rigorously tested.

Photoprotective detergent If you don’t want to purchase UPF clothing, an alternative would be washing your clothing with a laundry additive to boost its UV blocking ability. For more information, visit the Skin Cancer Foundation website.

Do I need to be exposed to sunlight to prevent vitamin D deficiency and does sunscreen use cause vitamin D deficiency? 

The answer to this question is complicated and remains an area of much debate within academic circles. Ultraviolet radiation has been officially designated as an environmental carcinogen, unquestionably associated with the development of skin cancers. Ultraviolet radiation also stimulates the skin to produce a precursor of vitamin D, called previtamin D3. Since vitamin D has been shown to prevent osteoporosis, and possibly lower blood pressure, reduce mortality from some cancers and prevent type 1 diabetes, this raises the question, does the use of sunscreen inhibit maintenance of recommended vitamin D levels and therefore, its benefits. All available evidence indicates that younger, lighter-skinned individuals easily maintain recommended vitamin D levels by incidental protected sun exposure and diet. In the United States, most of our foods are fortified with calcium and vitamin D. Vitamin D is available in dietary supplements and foods such as salmon and eggs, as well as enriched milk and orange juice. Increasing the skin’s production of previtamin D3 through intentional unprotected sun exposure does not offer concomitant calcium supplementation and is known to be inefficient in those at highest risk for vitamin D deficiency, the elderly and dark-skinned persons. For high risk individuals and those who are vitamin D deficient, supplemental vitamin D (200-1000IU) with dietary calcium can be taken to meet current RDA levels (2).

End of the Sunscreen and Melanoma Controversy

Sunscreens were developed to protect against the damaging effects of UV rays: sunburns, skin cancers and photoaging. However, there have been several misleading reports that suggested sunscreen use may cause melanoma (3-5). A person’s Fitzpatrick skin type, how sensitive someone is to the sun based on skin color, confounds the association between sunscreen use and melanoma, yet these studies failed to appropriately control for this. Sun-sensitive persons are more likely to use sunscreens and are at higher risk for melanoma. Thus, these reports may reflect an increased risk among sun-sensitive persons rather than an increased risk due to sunscreen application. A meta-analysis of 18 studies published from 1966 to 2003 that reported information on sunscreen use and melanoma found no good evidence for an increased risk for melanoma with sunscreen use (6).

References

1.      Edlich RF, Cox MJ, Becker DG et al. Revolutionary Advances in Sun-Protective Clothing—An Essential Step in Eliminating Skin Cancer in our World. Journal of Long-Term Effects of Medical Implants 2004;14(2)95–105

2.      Wolpowitz D, Gilchrest BA. The vitamin D questions: How much do you need and should you get it? J Am Acad Dermatol 2006; 52: 301-317

3.      Westerdahl J, Ingvar C, Masback A, Olsoon H.  Sunscreen use and malignant melanoma.  Int J Caner.  2000;87:145-50.

4.      Wolf P, Quenhenberger F, Mullegger R, Stranz B, Kerl H.  Phenotypic markers, sunlight-related factors and sunscreen use in patients with cutaneous melanoma: an Austrian case-control study.  Melanoma Res.  1998;8:370-8.

5.      Whiteman DC, Valery P, McWhirter W, Green AC.  Risk factors for childhood melanoma in Wueensland, Australia.  Int J. Cancer.  1997;70:26:31.

6.      Dennis LK, Freeman LB, VanBeek M.  Sunscreen use and the risk of melanoma:  A Quantitative Review.  Ann Intern Med.  2003; 139:966-978.

 

Resources

http://www.aad.org/public/publications/pamphlets/sun_sunscreens.html

http://www.melanomafoundation.org/

http://www.cancer.gov/cancertopics/pdq/prevention/skin/HealthProfessional/page4

http://www.skincancer.org

Suraj Venna, MD, FAAD Director, Melanoma Center Washington Cancer Institute 110 Irving Street, NW Washington DC 20010 (o) 202 877 2551

Felix Kuo, MD, FAAD 901 Stewart Avenue Suite 201 Garden City, NY 11530 (516) 227- 3377