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By David Warmflash

David WarmflashBeach season is here, which usually means incoming waves of stories about studies relevant to the sunscreen issue. This year is no different, since a report published recently by investigators at the Texas Biomedical Research Institute has been in the news, and should be on your mind as you head to the coast this summer.

Using an opossum model, the study demonstrates a 10-fold reduction in pre-melanotic lesions in adult opossums treated with sunscreen on a regular basis during the infant stage. This translates into a high-risk reduction for malignant melanoma—the worst kind of skin cancer—for humans if they use sunscreen on a regular basis as young children. The phrase “on a regular basis” is vital.

But to interpret the results, we need to put on our thinking caps.

While it has been clear for some time that sunscreen lotion can prevent sunburn and while the American Academy of Dermatology recommends sunscreen, the epidemiology connecting sunscreens and melanoma risk has been debated in recent years. A big study published in 2011, in which 1,621 people in Australia had been followed for many years, found a clear reduction in the risk of cutaneous malignant melanoma (CMM) for those who used sunscreen regularly. But for that study regularly means through the day, everyday, throughout the year, even in the winter.

Given the fact that exposure to solar ultraviolet (UV) radiation is different at the latitudes of many North American cities compared with Australia, people at high latitudes are not as cognizant of being in the sun during winter. Consequently, they are unlikely to use sunscreen during winter, when most of
their skin is covered by clothes anyway. Since the epidemiologic advantage in the Australian study resulted from constant sunscreen use, a review later that year in the Journal of the American Board of Family Medicine (JABFM) examined a range of studies looking at the CMM risk from several angles. This included review of studies that had analyzed the CMM risk in different North American cities separately. It turns out that when the CMM risk is assessed relative to sunscreen as it’s used by most people—putting it on at times when you’re very aware of being in strong sunlight—there was a slight increase in the CMM risk for those using sunscreen with a high sun protection factor (SPF). Those using sunscreen with an SPF of 15 or below did not show an increased risk based on the studies reviewed in the JABFM paper, but sunscreen with SPF 30, 50, or more (meaning the kind we are told to use) seems to be associated with a slight increase in CMM.

Now that seems odd, until you consider possible reasons for the association. One very controversial possibility is that possible negative effects that zinc nanoparticles and other agents in sunscreens could outweigh the benefit of blocking UV light. Rather than make an issue of this possibility, however, the JABFM review focused on issues related to vitamin D, the types of UV radiation blocked by different sunscreens, and sunscreen habits. In doing so, the author, Margaret B. Planta, introduced a perspective that is just as applicable today in the context of the new opossum study.

Vitamin D is made by the skin’s melanocytes, the same cells that can develop into CMM, and it protects against the development and proliferation of CMM and other cancers. The synthesis of vitamin D requires UV light, raising the possibility that blocking the UV light with sunscreen could raise the risk of cancer indirectly by lowering the level of vitamin D. When encountered gradually, the UV light also stimulates the production of the pigment melanin. This darkens the skin, which is what sunbathers want, but it also protects the skin by reducing the amount of UV that gets in. In fact, that’s why melanin evolved.

BoyReceivingSunscreenBased on the wavelength, UV is categorized into three ranges. UVC is blocked by the atmosphere; so it’s an issue only in spaceflight. UVB is partly blocked but some gets through, and it’s more dangerous than the lower energy UVA, which gets through the atmosphere very easily. All sunscreens block UVB, but the UVA plays an important role in the development of CMM. So sunscreens with broad spectrum UV protection (UVA and UVB) are better, while the UVB-only sunscreens could be increasing the CMM risk by allowing the person to stay in the sun longer without burning.

The most complicated factor, however, is people’s sunscreen habits. The UV blocking property doesn’t last so long; sunscreens wear off. To be effective, they must be applied repeatedly, but not all people may be doing this. So putting on the sunscreen initially, whether on your children or yourself, could give you a false sense of security, encouraging you to stay in the sunlight longer. To avoid this problem, it may be easier to use a kind of sunblock that doesn’t wear off, namely a big old hat.  So maybe our thinking caps can help us do more than merely interpret the results of this new study.

David Warmflash, M.D., is an astrobiologist, science writer, and physician. He is principal investigator on a Planetary Society-sponsored investigation of the effects of the space environment on organisms.