On the face of it, Sun Protection Factor, commonly known as SPF, seems simple. Every sunscreen is assigned an SPF number. This number is the factor by which the amount of time it takes for sunburn to occur is increased when sunscreen is used versus when skin is left unprotected. It is an important measure, but it’s not all there is to know about sunscreen, and this is a case where what you don’t know can hurt you.
While SPF is a useful number, it is neither an absolute value nor is it simple. It is determined under laboratory conditions that may or may not mimic the way individuals use sunscreen in the real world (individuals are unlikely to apply sunscreen in the amount used for laboratory testing). To further complicate an understanding of this number, when efficacy is considered relative to the percentage of energy that is reflected or absorbed, SPF does not provide a linear scale. And, perhaps most importantly, SPF is an indicator of only one type of protection. Current SPF rating does not adequately measure protection from all the damaging radiation effects of light, just UVB.
While regulatory guidelines do exist, they are problematic. In the United States, the testing guidelines established by the Food and Drug Administration (FDA) only account for UVB protection. Unlike other countries, the U.S. FDA has not established specific guidelines to test a product’s protective ability relative to UVA, which is known to cause cancer and other skin damage. Whether done in the United States or in another country, testing to determine SPF is agreed upon by a uniform central body, but actual testing is decentralized. This means there may be variations in how guidelines are interpreted and they may be applied inconsistently. As a case in point, recent reports issued by Which?, a consumer watchdog magazine published in the UK, have raised serious questions about the accuracy of SPF ratings listed on European sunscreen labels. Using the test endorsed by the European Union (EU) recommendations on sunscreen labelling, the magazine Which? annually tests the SPF of sunscreens to strict international standards and compares the resulting information to product label claims. For these tests, sunscreen is applied to a small test area on the backs of 12 volunteers who are exposed to UVB rays from a special lamp that stimulates sunlight and records when the skin turns red. In both 2008 and 2009, the magazine Which? found that although a few sunscreens exceeded the SPF rating on their labels, many offered a lower SPF than the labelling indicated. In 2008, four sunscreens labelled as SPF 15 were found to only provide an SPF of 9.3 to 12.1.
Although the FDA has not yet established guidelines to test a sunscreen’s protective ability relative to UVA, under their new labeling requirements for over-the-counter sunscreens, any product labeled “Broad-Spectrum SPF [value]” must pass a broad-spectrum test demonstrating that it provides UVA protection that is proportional to its UVB protection. Thus, sunscreens labeled as “broad-spectrum” offer a higher level of protection from both UVB and UVA radiation. [FDA-Federal Register OTC Sunscreen Labeling; p15-16]
In relative scales, SPF means the percentage of sunlight that is absorbed or reflected due to the level of the SPF. For example, an SPF with a value of 100 would reflect or absorb 99% of UV rays, whereas an SPF with a value of 10 reflects or absorbs 90%. The scale is not a linear scale; rather it is derived by the way SPF is calculated.
Finally, the technical explanation of the benefits of 30+ SPF is complicated. For instance, an SPF 15 product blocks 93% of incidental UVB light, while SPF 34 blocks 97%, a seemingly insignificant difference. Nevertheless, a dermatologist in Philadelphia, Pennsylvania completed an instructive model using human skin volunteers showing that skin protected with an SPF 30 sunscreen had 2.5 times fewer sunburn damaged cells than skin protected with an SPF 15 sunscreen when both areas were exposed to enough simulated solar radiation to cause the beginning of redness.i Thus, there is at least a two-fold difference in the protection offered by the higher SPF.
The Facts of Light IV:
The Truth About SPF
- SPF merely addresses the UVB wavelength not UVA, UVC, visible, or infrared light.
- The accuracy of the SPF number is questionable as testing is done with more product than is realistically used when sunscreen is applied. Consider the reality that the true SPF number is about 1/2 or 1/3 of the stated number.
- Using a higher SPF sunscreen can help overcome “user-errors”, e.g. sporadic use of product, not reapplying frequent enough or in an adequate amount.
- When utilizing titanium dioxide and zinc oxide, the higher the SPF the more UVA coverage.
- The UVA ray is more difficult to study on human subjects due to the length of time it takes for damage to appear. Aging skin or mutating skin cells are cumulative and can take decades to manifest fully.
Sunburn At a Glance
- Ultraviolet B rays have long been known to harm the skin.
- Sunburn is inflammation of the skin due to overexposure to ultraviolet (UV) rays. Sunburn can be permanent due to damage being cumulative.
- Sunburn can be serious and require professional medical attention.
- Victims of severe sunburn should avoid bathing in cold water.
- Many prescription and non-prescription drugs and products increase the skin’s sensitivity to sunlight.
- The UV from “sun tanning” lamps is as damaging to skin as sun exposure.
- The main environmental cause of skin cancer is the sun.