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Basically, sunscreen users are in the sun for much longer. So simply comparing those who don’t use sunscreen at all to those who use low factor is not relevant, because the two groups don’t spend equal amounts of time in the sun.
“The explanation for this paradox is that some people use sunscreen to prolong sun exposure and acquire suntan. Moreover, many people don’t apply the proper amount of sunscreen, forget to reapply and missed to apply on all exposed areas resulting in sunburn and increased risk of melanoma,” said Reza Ghiasvand, a PhD candidate at The Department of Biostatistics and a member of the research group “Epidemiological Studies of Lifestyle and Chronic Diseases.” “We found that those who used sunscreen with a factor higher than 15 had a 33% lower risk of melanoma compared with those using sunscreen with a low factor.”
So the takeaway is simple: if you’re in the sun a lot, use high factor sunscreen and don’t take any chances.
Journal Reference: R. Ghiasvand, E. Weiderpass, A. C. Green, E. Lund, M. B. Veierod. Sunscreen Use and Subsequent Melanoma Risk: A Population-Based Cohort Study. Journal of Clinical Oncology, 2016; DOI: 10.1200/JCO.2016.67.5934