J. Cosmet. Sci., 71, 191–196 (July/August 2020) 191 Regular Application of Sunscreen Can Prevent Skin Cancer ADÈLE C. GREEN, Population Studies Department, QIMR Berghofer Medical Research Institute, Brisbane, 4006 Australia, C RUK Manchester Institute and Faculty of Biology, Medicine and Health, University of Manchester, Manchester Academic Health Sciences Centre, Manchester, M13 9PL United Kingdom (A.C.G.) Synopsis This review summarizes the evidence on the protection against skin cancer afforded by sunscreen. Solid evidence can come only from randomized controlled trials, despite a multitude of case–control and cohort studies that have addressed the issue, because observational evidence is intractably confounded since those at highest risk of skin cancer are naturally the highest users of sunscreen. Findings of the single human trial conducted in subtropical Australia during 1992–1996 with follow-up to 2014 showed that the application of a broad-spectrum, sun protection factor 16 sunscreen to exposed skin of the head and neck and upper limbs at least 3–4 days per week in adulthood can reduce the risk of developing cutaneous squamous cell carcinoma and melanoma but does not appear to reduce the risk of basal cell carcinoma (BCC) overall, although it may reduce the occurrence of multiple BCCs over time. Skin cancers constitute the most common types of cancer in predominantly white-skinned populations. There are three major types of skin cancer—the most common is basal cell carcinoma (BCC), with squamous cell carcinoma (SCC) the second most common and more serious because of its propensity to metastasize, and the least common but poten- tially fatal if not treated early is melanoma. Together, these cancers impose a costly bur- den on affected populations because of the extensive healthcare resources needed to treat them. Personal costs are also substantial and include cosmetic as well as out-of-pocket costs because skin cancer affects the face most frequently followed by other body sites that are often or occasionally exposed (1). High exposure to solar ultraviolet (UV) radiation, the shortest wavelength component of sunlight on earth, is a cause of all three types of skin cancer (2), although the exact pattern and total amount of sun exposure required differs for each type (1,2). Thus, mainly white- skinned populations are most susceptible to skin cancer, likely because they lack the UV-shielding melanin skin pigment possessed by dark-skinned populations (see the article by Antony Young in this issue). High UV levels also explain why the continually exposed skin of the face, head and neck in general, and forearms and hands are the body sites most Address all correspondence to adele.green@qimrberghofer.edu.au.

JOURNAL OF COSMETIC SCIENCE 192 affected. The most straightforward way to prevent skin cancer is to avoid intense sun exposure of the skin. Measures include staying indoors in the middle part of the day when ambient UV levels peak or seeking shade and wearing protective clothing, such as hats and long sleeves, when outdoors (3). The application of sunscreen that shields the skin by refl ecting or fi ltering UV radiation is another popular measure. This article reviews the available evidence regarding the use of sunscreens for skin cancer prevention. EVIDENCE FROM OBSERVATIONAL STUDIES One of the most extensive reviews of relevant observational studies was a recent system- atic review of the evidence in humans (available to February 2018) regarding the use of sunscreens for melanoma prevention (4). The authors identifi ed 23 case–control, one eco- logical, and three cohort studies, and their evidence was deemed weak and heterogeneous because of the challenges of controlling for “confounding by indication” (4). This con- founding occurs because factors such as sunburn susceptibility, high occupational or rec- reational sun exposure, or previous skin cancer determine both sunscreen use and skin cancer risk (5), so a priori people who use sunscreen are naturally the people most likely to develop skin cancer, and this cannot be adjusted for in analysis of observational studies. Randomized controlled trials (RCTs) are the only kind of study that avoid this confound- ing through the random allocation of sunscreen use irrespective of the risk of skin cancer and, therefore, the only kind of study that can properly evaluate the long-term effective- ness of sunscreen in preventing skin cancer (5). EVIDENCE FROM RCTS There has been only one RCT that has assessed the effects of using sunscreen on skin cancer occurrence. It was conducted in Nambour, a township in subtropical Queensland, Australia (6). The participants were initially selected at random from a community reg- ister (the electoral roll: voting is compulsory in Australia) in 1986 when they were aged 20–69 years, for a study of skin cancer prevalence followed by incidence studies (7). Of the 2,095 people in the original prevalence study, 1,621 consented to take part in the fi eld trial that evaluated daily application of sunscreen (and daily oral supplements of beta-carotene) to prevent skin cancer, from 1992 to 1996 (6). At baseline in 1992, they completed self-administered questionnaires recording personal characteristics such as education, smoking, medical conditions, and medications, and skin cancer risk factors, namely, skin color, sunburn tendency and past sunburns, and occupational (weekday) and recreational (weekend and holidays) sun exposure (6). All received full-body skin exami- nations for skin cancer and other signs of sun damage by dermatologists at trial baseline. Using a computer-generated random assignment sequence, trial participants were allo- cated to one of four intervention groups: daily application of a standard sunscreen (details in the following paragraph) and a 30-mg beta-carotene tablet each day, daily sunscreen and placebo tablet, beta-carotene only, and placebo only (8). (The beta-carotene interven- tion had no effect on skin cancer and is not discussed further.) Participants not assigned to daily application of sunscreen were not assigned placebo sunscreen for ethical reasons and were asked to continue the use of sunscreen at their discretion, which was mostly recreational use or no use.