Is a sunscreen with an SPF of 30 really no better than one with an SPF of 15? Do sunscreens prevent...or cause skin cancer? Is sunscreen use causing vitamin D deficiency? Despite how ubiquitous sunscreens are, studies have shown that many of us actually know very little about the lotions we put on to protect us from the sun. The following are some of the sunscreen "myths" I've encountered, along with my attempt to clarify what it is that the science tells us.
1) Myth #1: SPF 30 is no better than SPF 15
It is true that under laboratory conditions, a sunscreen with an SPF of 15 will filter out 93% of the ultraviolet rays that cause sunburn, while a sunscreen with an SPF of 30 will filter out 97%. In real life situations, however, the majority of people do not apply enough sunscreen to achieve the SPF a product can provide. An SPF value is calculated based on the application of 2 mg/cm 2 of sunscreen; a generous quantity that, unfortunately, most people don't use. In fact, studies have shown that people generally apply 20-50% of the amount of sunscreen that is recommended (Autier 2001, Neale 2002). Using a sunscreen with an SPF of 30 improperly, and achieving an SPF of 10, is far better than the SPF of 5 that might result from inadequate application of a sunscreen with an SPF of 15.
Even under ideal circumstances in which someone uses 2 mg/cm 2 of sunscreen, using a product with an SPF of 15 will allow 7% of the radiation that causes sunburn from reaching the healthy layers of the skin, while using one with an SPF of 30 will allow only 3% through. That's more than double the radiation that gets past the sunscreen and reaches the skin's living cells. A study using a simulation model of how people apply sunscreen found that, when sunscreen is applied unevenly and in inadequate quantities (as it often is), using a sunscreen with an SPF of 15 on a day in which the UV index is high, would result in more than twice the extent of sunburn than would occur if a sunscreen with an SPF of 30 were used (Pissavini 2013).
The bottom line: use a sunscreen with an SPF of 30 (or higher). You'll get better sun protection with more room for (inevitable) human error.
2) Myth #2: SPF = UVB
The SPF stands for sun protection factor. It is a measure of how well a sunscreen protects you from sunburn. A sunscreen with an SPF of 15 should, if applied properly, allow you to stay in the sun 15 times as long before you burn.
The ultraviolet radiation (UVR) that reaches the earth from the sun is divided in to UVA and UVB. UVA rays are further divided in to UVA1 and UVA2. UVB rays are shorter than UVA rays and are more energetic, and they bear more responsibility for causing both sunburn and skin cancer. UVA rays are much more plentiful; about 20 times more UVA radiation than UVB reaches our live skin cells. UVA rays are longer and they penetrate the skin more deeply. In addition to being carcinogenic in their own right, these rays are the primary cause of skin aging. Protection from both UVA and UVB is important.
Sunscreens were originally developed to allow people to tan without burning, and as such they did a much better job of protecting against the shorter UV rays that are more erythemogenic (more likely to cause sunburn). It wasn't until much later that scientists realized that the longer UVA rays that cause tanning are also harmful, and most modern sunscreens now offer protection that is "broad-spectrum".
It has been estimated that up to 20% of the solar radiation that causes sunburn is a result of radiation from UVA (primarily shorter UVA2) rays; the remaining 80% is the result of the more energetic UVB rays (CIE 1999, Lim HW 2007). In fact, it has been calculated that it is mathematically impossible to achieve an SPF higher than 11 without filtering some UVA rays in addition to UVB rays (Sayre 2008). While a higher SPF value offers some indication of whether a sunscreen protects from shorter UVA rays , on its own it poorly reflects protection from the longest, and most plentiful UVA rays (known as UVA1). Sunscreens that carry the label "broad spectrum" will provide some protection from these longer rays.
The bottom line: Choosing a sunscreen with a high SPF will provide you with protection from UVB and some shorter UVA rays. Look for a sunscreen that is "broad-spectrum" to ensure your sunscreen protects from a larger portion of the UVA spectrum. See here for more information on how to choose a sunscreen.
3) Myth #3: Sunscreens cause skin cancer
I have, unfortunately, come across this myth on the internet. Sunscreens prevent skin cancer, they do not cause it. We now have high-quality scientific evidence that has shown us that the regular use of a broad-spectrum sunscreen can reduce the risk of skin cancer. A large, randomized controlled trial was conducted in Australia over a 4.5-year period in order to examine this issue. At the conclusion of this study, participants who had been randomized to daily sunscreen use had developed 39% fewer squamous cell carcinomas than those who had been randomized to using sunscreen at their discretion (Green 1999). Long-term follow-up of the participants in this study over an additional 10 years found, that those who had been randomized to daily sunscreen use also had a 50% reduction in their risk of melanoma, and a 73% reduction in the risk of Invasive melanoma (the more dangerous type of melanomas) (Green 2011).
There were a few early studies that showed that, when asked about their history of sunscreen use, people who had melanoma reported having used sunscreen more regularly. There are many problems with these studies. Asking people to remember how often they used sunscreen decades ago is fraught with room for error; people often don't remember what they did last week, let alone 20 or 30 years ago. Additionally, fair-skinned people who are at higher risk for melanoma are also more likely to use sunscreen to avoid burning, making it more likely that their skin colour contributed to their developing melanoma rather than their sunscreen use. Finally, these older studies may not be relevant today because sunscreens in use at the time were not nearly as effective as those we use now. Early sunscreens had low SPF values and offered poor protection against UVA rays.
In order to answer this question, two meta-analyses (studies that review many other studies) of the subject were published in 2002 (Huncharek 2002) and 2003 (Dennis 2003). Both concluded that the evidence did not support a role for sunscreen use in causing melanoma.
The bottom line: Sunscreens do not cause skin cancer.
4) Myth #4: Regular sunscreen use has been associated with vitamin D deficiency
It is true that vitamin D is converted into an active form in the skin, and that this process requires UVB (ultraviolet B) radiation. It is also true, however, that studies of people who use sunscreen in real-life circumstances have found no significant difference in vitamin D levels between those who use sunscreen regularly and those who do not (Marks 1995, Harris 2007). Other studies have shown a trend toward increasing vitamin D levels in those who use sunscreen, likely because people who use sunscreen spend more time outdoors (Kligman 1989, Kimlin 2007).
Sunscreens probably do not suppress vitamin D production in real-life circumstances because people do not use enough sunscreen for that to happen. In fair-skinned people, brief periods of unprotected sun exposure in the spring and summer usually result in adequate vitamin D synthesis, while excess exposure dramatically increases skin cancer risk. For those who are religious about sun protection and are concerned about their vitamin D levels, vitamin D supplements are an inexpensive, and widely available alternative to excess sun exposure. Speak with your primary care doctor about whether these are right for you.
The bottom line: Studies have shown that people who are more likely to use sunscreen are not more likely to be deficient in vitamin D. Limit unprotected sun exposure and avoid sunburn. Speak with your doctor if you are concerned about vitamin D deficiency.
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Sayre RM, Dowdy JC, Lott DL, Marlowe E. Commentary on 'UVB-SPF': the SPF labels of sunscreen products convey more than just UVB protection. Photodermatol Photoimmunol Photomed 2008: 24: 218-220.