Linking the ultraviolet index to the shadow rule for sun protection : a theoretical model applied with a southern hemisphere example

Abstract

Personal solar ultraviolet radiation (UVR) exposure has positive and negative impacts on human health. Excess solar UVR exposure can be avoided through safe sun practices such as using sun protection and avoiding unprotected outdoor exposure when solar UVR levels are high. The shadow rule indirectly determines the sun's altitude by observing the length of a person's shadow during the course of the day. When the shadow cast by the sun on a horizontal surface is shorter than the height of the person casting the shadow, the solar UVR intensity is high and is deemed to have more risk. The magnitude of this risk depends on factors such a skin type, sun protection used etc. The UV Index is a standardized measure to describe the intensity of solar UVR with respect to the human action spectrum for sunburn. It is frequently reported in weather forecasts aimed at the public. Here, we demonstrate the potential utility of the shadow rule and how it may be understood in relation to the UV Index in a subtropical southern hemisphere setting. Its use as a simple awareness tool for sun protection in locations where the UV Index is not made public has value in promoting sun exposure awareness and reducing personal exposure risk.