Studies Shed Light on Sunscreen Efficacy

Environmental Health Perspectives (May 1998)

The relationship between use of sunscreen and prevention of skin cancer remains unclear despite intriguing results of several studies presented on 17 February 1998 at the American Association for the Advancement of Science annual meeting in Philadelphia, Pennsylvania. According to these studies, consumers can't be certain which sunscreen, if any, will lower their risk for any of the three types of skin cancers. Two preliminary studies even suggest that using sunscreen may increase cancer risk.

Sunscreens are formulated to protect against sunburn, and, though a prophylactic benefit has long been assumed by both the public and academia, there is little evidence that preventing sunburn in human skin prevents skin cancer. It is well-established that 90% of skin cancers are caused by exposure to light, but the causal mechanisms for basal cell carcinoma (BCC) and squamous cell carcinoma (SCC)--both known as nonmelanoma cancers--and melanoma are only now being probed. In the United States, according to Marianne Berwick, an associate attending epidemiologist at the Memorial Sloan-Kettering Cancer Center in New York City, there are approximately 1 million new cases of non-melanoma skin cancer annually with about 1,200 deaths, and about 40,000 new melanoma cases with 7,200 deaths. Melanoma metastasizes readily, while the nonmelanomas rarely do.

False sense of security? New information shows that use of sunscreen may not be enough to protect against skin cancer, and people's dependence on them could actually lead to increased risk.

According to session organizer Francis Gasparro, director of the Jefferson University Photobiology Laboratory at Thomas Jefferson University in Philadelphia, 21 FDA-approved compounds are potentially available for use in sunscreens marketed in the United States. However, most of the research into their photochemistry has been done by industry, and the results are not available to either the public or academic researchers. Many sunscreens protect against some part of the ultraviolet (UV) spectrum, Gasparro says, but none of the sunscreens available perform "like a layer of concrete on your skin."

Hoping to unravel the connections between melanoma, long-wave ultraviolet radiation (UVA), and short-wave ultraviolet radiation (UVB), biophysicist Richard Setlow of the Brookhaven National Laboratory in New York exposed light-sensitive tropical fish to UVA. He found a high incidence of melanoma induction. Setlow suggests that if the fish results are transferable to humans, sunscreens formulated to block only UVB do not offer reasonable protection against melanoma.

Two studies reported at the February meeting sought to determine whether sunscreen protects DNA from UV damage. In a study funded by a consortium of pharmaceutical and cosmetics manufacturers, Honnavara Ananthaswamy, professor and deputy chairman of the department of immunology at the University of Texas M.D. Anderson Cancer Center in Houston, and his team tracked the rate of mutation in the p53 tumor suppressor gene in mice exposed to UVB. Results of the study, published in the May 1997 issue of Nature Medicine, showed that after 16 weeks, in mice pretreated with sunscreen with a sun protection factor (SPF) of 15, p53 mutations were almost nonexistent, Ananthaswamy reports, whereas 50% of the mice without sunscreen showed the mutation after 12 weeks. All of the mice without sunscreen developed skin tumors after 41 weeks of daily exposure. None of the mice treated with sunscreen developed skin tumors during this time or even after 54 weeks of continuous sunscreen and UV exposure. Ananthaswamy says the p53 mutation can serve as a very early warning of nonmelanoma skin cancer induction. He would like to see what he calls an "MPF" (mutation protection factor) added to the SPF designation on sunscreens.

John Knowland, a researcher in the department of biochemistry at Oxford University in the United Kingdom, studied whether sunscreen compounds become chemically reactive in the presence of UV light and pass their excess energy to DNA. In his study, published in the August 1997 issue of Photochemistry and Photobiology, Knowland exposed both naked DNA and cultured human cells treated with padimate O, a derivative of para-aminobenzoic acid, to UV radiation in the laboratory. In both cases he observed DNA strand breakage, presumably caused by hydroxyl radicals.

Thus, one suppressor gene study shows promise for sunscreens in the prevention of nonmelanoma skin cancers, and the other shows that at least one sunscreen component itself actually induces DNA damage. The epidemiological evidence is equally confusing. Berwick surveyed 16 epidemiological studies, and says that these studies show that "squamous cell carcinoma is associated with continuous sun exposure, basal cell carcinoma seems to be associated with continuous [and] intermittent sun exposure on the unadapted skin, and melanoma seems to be associated with intermittent, intense sun exposure on untanned, unadapted skin."

Two SCC studies found that sunscreen did protect against precursor lesions. The other 14 studies are "extremely mixed," Berwick says. Two BCC studies found a positive association between the use of sunscreen and the incidence of BCC. Of 10 melanoma studies, five showed a positive association between the use of sunscreens and the development of melanoma. Two showed sunscreen to be protective, and three showed no association.

"We can conclude from these studies that it is not safe to rely on sunscreen to protect you from getting skin cancer," Berwick says. She notes that the positive association between sunscreen and melanoma may be due to the possibility that for people at highest risk for developing melanoma (light-skinned, light-eyed people, especially those with many moles), sunscreen may bestow a false sense of security. They may stay out in the sun longer than they would otherwise. Berwick emphasizes that until more clarifying research is done, people should pay close attention to their skin-cancer risk factors and reduce their sun exposure accordingly.

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