Implications of Low Levels of the UV Index for Sun Protection

A Global Solar Ultraviolet Index (UVI) value of 2 is generally linked to the health message 'You can safely stay outside!' To examine whether this is sound advice for all skin types and even for prolonged periods spent outside we used erythemal irradiance data of all 136 days during the study period from 2014 till 2016 with such a UVI measured by the German Federal Office for Radiation Protection (BfS) in Munich, Germany. A comparison between the ambient erythemal doses calculated for various time intervals and minimal erythemal doses (MEDs) of the Caucasian skin types I-IV led us to a critical reappraisal of the above health message. Specifically, the message might be misleading if people with a fair complexion want to spend several hours outside, because without any protective measures the doses received can be sufficient to induce erythema. We thus recommend an amendment of the health message related to a safe level of the UVI and, moreover, generally tailoring UVI-related health messages to different skin types. Currently, these messages do not seem to be strictly evidence based, which might be one reason for the unexpected result of our analysis.