Sunscreens and Skin Cancer – the Science
The first sunscreens were developed to prevent severe sunburn in military personnel who spend long hours under strong and direct sunlight (Maceachern 1964). Today, their makers claim all sorts of benefits, from preventing skin aging and damage to protection from skin cancer. Yet experts disagree on the strength and reliability of the scientific evidence behind these claims (Autier 2009, Draelos 2010).
Effectiv December 2012, sunscreens that meet specific FDA criteria will be allowed to make this claim:
“If used as directed with other sun protection measures, decreases the risk of skin cancer and early skin aging caused by the sun” (FDA 2011a).
The FDA decided to allow this claim because sunscreens block UV radiation, which is known to cause skin cancer. Yet FDA has also cited studies showing that sunscreen users spend more time in the sun than non-users. This suggests that the claim of cancer reduction may depend more on the amount of time spent out of the sun than on sunscreen use.
The power of sunscreens to protect against sunburn is well established. This is the feature of sunscreens identified as the Sun Protection Factor or SPF. Yet sunscreens allow people with fair skin to stay outdoors longer, often to get a tan or to maximize burn-free time in the sun. In the process, they are intensely exposed to UVA rays, which do not cause burning but inflict more subtle damage (Autier 2009, Lautenschlager 2007). Meanwhile, research findings conflict.
Skin cancer is the most common cancer in the United States, accounting for nearly half of all cases. The latest statistics raise questions about most effective ways to avoid skin cancer:
- More than two million Americans develop skin cancer each year (Bikle 2008, Rogers 2010, ACS 2010).
- Skin cancer is five times more prevalent in the U.S. population than breast or prostate cancers (Stern 2010).
Even though more people are using sunscreens than ever before, and products are improving in quality, the incidence of skin cancer in the United States and other countries continues to rise (Aceituno-Madera 2010, Jemal 2008, Osterlind 1992). In fact, a number of studies conducted in the 1990s reported higher, not lower, incidence of the deadliest form of skin cancer, malignant melanoma, among frequent sunscreen users (Autier 1998, Beitner 1990, Westerdahl 2000, Wolf 1998). But other studies suggest that sunscreens protect against melanoma (Autier 1995, Green 2010, Westerdahl 2000, Wolf 1994).
Studies show that regular sunscreen use reduces the risk of squamous-cell carcinoma (SCC) but not necessarily of other types of skin cancer. Squamous-cell cancer, a slow-growing, treatable disease, is estimated to account for just 16 percent of all skin cancers. Of the remaining cancers, 80 percent are basal-cell carcinoma and 4 percent are malignant melanoma, according to a review published by the American Cancer Society (Greelee 2001). The International Agency for Research on Cancer (IARC) has concluded that:
- Sunscreen use may decrease the occurrence of squamous cell carcinoma.
- Sunscreen use has no demonstrated influence on basal cell carcinoma.
- In intentional sun exposure situations such as staying outdoors for long periods of time, sunscreen use may increase the risk of melanoma (IARC 2001a, Autier 2009).
Melanoma and sunscreen: UVA, UVB, or both?
Sunlight that reaches Earth’s surface consists of longer-wavelength UVA (315–400 nanometers -nm), shorter wavelength UVB (280–315 nm), visible light and infrared light. UVB constitutes 3-5 percent of the total UV radiation that gets through the atmosphere, while UVA constitutes 95-97 percent.
UVB, which penetrates only the outer skin layer, is the primary cause of sunburn (skin erythema or redness) and non-melanoma skin cancers such as squamous cell carcinoma (von Thaler 2010). UVA penetrates deeper into the skin, causing a different type of DNA damage than UVB (Cadet 2009).
For decades, sunscreen manufacturers and sunscreen users assumed that preventing or delaying sunburn would also avert other dangerous damage, such as skin cancer. Today, many experts believe that both UVA and UVB exposure may contribute to melanoma risk (Donawho 1996, Garland 2003, Godar 2009, Setlow 1993). Sunscreens sold over the past three decades may not have afforded cancer protection because they blocked UVB but allowed higher UVA exposure (Draelos 2010). Thus sunscreens may have contributed to risk of melanoma in some populations (Gorham 2007).
Even though many sunscreens on the U.S. market now contain one of three UVA filters, a large number of products available in 2012 still offer less than optimal protection. This problem is expected to continue until FDA approves better UVA filters and toughens its standards for UVA protection.
A growing body of data points to UVA exposure as a significant risk factor in melanoma development:
– Results from animal, epidemiological and clinical studies “suggest that UVA may play an important role in the pathogenesis of malignant melanoma” (Rünger 1999, Photodermatology, photoimmunology & photomedicine).
– “Collectively, [current] data suggest a potential role for UVA in the pathogenesis of melanoma” (Wang et al 2001, Journal of the American Academy of Dermatology).
– “The [sunscreen's] ability to prevent sunburns (as measured by SPF) probably does not imply the ability to prevent melanoma or basal cell carcinoma” (Autier 2009, British Journal of Dermatology).
Why don’t scientists know more about sunscreen and skin cancer?
Three factors complicate the interpretation of studies of sunscreen efficacy:
- People wearing sunscreens tend to stay out in the sun longer, so their total dose of UV radiation, particularly harmful UVA rays, may be larger than in non-users (Autier 2000, Dupuy 2005, Stanton 2004).
- Early-generation sunscreens did not provide significant or adequate UVA protection and possibly even sufficient UVB protection (Diffey 2009, Lautenschlager 2007, Osterwalder 2009).
- Sunscreen use in the populations studied may not have been consistent or sufficient to protect against melanoma (Bech-Thomsen 1992, Thieden 2005).
Studies conducted over the past decade have confirmed that regular sunscreen use lowers the risk of squamous cell carcinoma (Gordon 2009, van der Pols 2006), confirming studies completed in the 1990s (Green 1999). Regular sunscreen application also diminishes the incidence of solar keratosis (also known as actinic keratosis), which are sun-induced skin changes that may become precursors to squamous cell carcinoma (Naylor 1995, Thompson 1993). For basal cell carcinoma, follow-up studies reported a slight and not-statistically significant decrease in risk associated with sunscreen use (Pandeya 2005, van der Pols 2006). Overall, for this cancer type, data on sunscreen benefits remain negative or equivocal (Hunter 1990, Rosenstein 1999, Rubin 2005).
However, physicians are most concerned about malignant melanoma, the deadliest type of skin cancer (Lund 2007, World Health Organization 2006). Sunburns are an important risk factor for melanoma (Leiter 2008). Intermittent, severe sunburns in childhood have been considered to pose the greatest risk, although sunburn throughout life likely contributes to melanoma development (Autier 1998, Dennis 2008).
State of the evidence on melanoma prevention
Individual studies provide conflicting evidence on the role of sunscreens in melanoma development. Studies in Sweden, Belgium, France, Germany, Austria and New York state have reported an elevated risk of melanoma in sunscreen users (Autier 1998, Beitner 1990, Graham 1985, Westerdahl 2000, Wolf 1998). In contrast, studies in Spain, Brazil, Australia and California reported decreased risk in sunscreen users (Bakos 2002, Espinosa-Arranz 1999, Green 2011, Holly 1995, Rodenas 1996).
A recent retrospective study asked more than 1,000 melanoma patients and age- and gender-matched controls in Minnesota about their history of sun exposure and sun protection. It found that both groups had low rates of sunscreen use over previous decades. There were only weak associations between sun protection and melanoma risk, but the relationship was slightly stronger for sun avoidance (i.e. physical measures such as hats, long sleeves and shade) than for sunscreen use (Lazovitch 2011).
Complicating the puzzle is recent research that sought to address the question of sunscreen efficacy for melanoma in a prospective study (Green 2011a). The study involved 1,621 sun-savvy volunteers in Queensland, Australia. Participants in the intervention group were counseled in sun protection and asked to wear a SPF 15+ sunscreen with good UVA protection daily, while members of the control group used sunscreen at their own discretion. Both groups exhibited a high level of sun protection, with 60 percent seeking shade and 75 percent wearing hats during the study period, and compliance rates of 40 percent and 67 percent during the follow-up observation (Green 2011b) Ten years after the trial ended, 11 new primary melanomas were diagnosed among the participants who used sunscreen daily, half as many as in the control group. This striking difference led researchers to conclude that “melanoma may be preventable by regular sunscreen use in adults” (Green 2011a).
Prior to these new publications, study results had been mixed. A 2000 IARC assessment of 15 studies found conflicting evidence on the possible link between sunscreens and melanoma. Three studies showed significantly lower risks of melanoma associated with sunscreen use, eight studies found significantly higher risks, and four studies found no effect (IARC 2001, reviewed in Dennis 2003, Diffey 2009, Gorham 2007, Huncharek 2002).
Some scientists have combined the data from multiple studies in what are called “meta-analyses” in order to assess larger or specialized groups of sunscreen users. A meta-analysis of melanoma studies conducted by University of Iowa scientists in 2003 found no overall association between melanoma risk and sunscreen use (Dennis 2003). The Iowa researchers suggested that findings of elevated risk in a large group of other studies conducted in Europe and U.S. may have been complicated by various factors, such as differences in skin sensitivity to sunlight among people with lighter or darker skin. Sunscreens are more likely to be used by people most at risk of quick sunburn (Diffey 2009, Geller 2002), a group that is also at higher risk of melanoma (Dubin 1986).
A meta-analysis conducted by University of California/San Diego scientists in 2007 found a link between the location of the study (distance from the equator) and the risk of melanoma in relationship to sunscreen use. According to this analysis, in populations living at latitudes 40 degrees or less from the equator, sunscreen use was not associated with a statistically significant risk of melanoma, while populations in more northern latitudes faced a statistically significant increase in melanoma risk linked to sunscreen use (Gorham 2007). This may be due to the greater relative prevalence of UVA than UVB radiation in northern latitudes.
Skin pigmentation may also be a factor in these latitude differences (Gorham 2007). Studies finding that sunscreens were protective generally included Mediterranean populations or populations with Mediterranean ancestry, whose skin has a higher degree of constitutive pigmentation. On the other hand, studies conducted among light-skinned populations residing far from the equator (above 40 degrees latitude) generally found a statistically significant 60 percent increase in melanoma risk (Espinosa Arranz 1999, Rodenas 1996). In other words, the ability of sunscreen to protect from melanoma appears to depend on the geographic location and the typical skin pigmentation of the population.
Extrapolating these results to residents at latitudes more distant from the equator – all of the US, Canada and Europe – will be difficult. One critique questioned the power of this study to detect statistically significant effects, given the small number of melanoma cases observed, and cautioned that “sunscreen use alone will likely not reduce the incidence of skin cancer” while continuing to recommend sunscreen use (Gimotty 2011).
Experts generally agree that the tendencies of sunscreen users to spend more recreational time in direct sunlight and to wear less protective clothing may exacerbate sun damage that leads to melanoma (Autier 2009, Draelos 2010, Gorham 2007). Scientists still do not know which specific wavelengths of sunlight drive melanoma development (Donawho 1996). Until recently, broad-spectrum UV protection, especially against UVA radiation, was lacking in sunscreens, a deficit that may have contributed to melanoma development, or at least to the lack of evidence for a decrease in melanoma risk for sunscreen users (Garland 2003, Godar 2009).
Since scientist do not have definitive answers about sunscreen and skin cancer, it is no wonder that many experts recommend that people rely on clothing and shade, rather than sunscreen, to protect themselves from sun exposure.