The Shadow Rule, the UV Index, and the 5S Steps in the Tropics

Solar ultraviolet radiation (UVR) induces the production of vitamin D (VD), but it also causes erythema and skin cancer among other diseases. The amount of UVR to get benefit and avoid harm is unknown. Some principles were proposed to provide accessible information on how to avoid overexposure to the Sun. They are (1) the Shadow Rule (SR); (2) the UV Index (UVI); and (3) the Slip, Slop, Slap, Seek, Slide (5S) steps. This work discusses these principles in the tropics, considering measurements of solar UVR, total ozone column, and cloudiness. Indeed, the SR leads to overexposures. The UVI is adequate for cloudless skies, but it does not consider radiation enhancement events by clouds. "Seek shade" in the 5S steps should be more emphasized as it is both the most efficient step in terms of avoiding direct sun and the friendliest step toward the environment. Therefore, photoprotection attitudes in the tropics should focus on the UVI and the 5S steps in addition to considering both VD sufficiency and environmental outcomes. The SR is clearly inaccurate.