Overexposure to ultraviolet (UV) radiation is the main modifiable risk factor for skin cancer. The Global Solar Ultraviolet Index (UVI) was introduced as a tool to visualize the intensity of UV radiation on a certain day which should enable and encourage people to take appropriate protective measures. The exposure category ‘low’ of the UVI, including values from 0 to 2, was linked to the health message ‘No protection required’ by the World Health Organization and collaborating centres. However, published evidence corroborating this advice is scarce. Therefore, we analysed ambient erythemal irradiance data of 14,431 daily UVI time series of low UVI days. Data were gathered at nine stations of the German solar UV monitoring network – covering all major climate areas in Germany – in the years 2007 - 2016. We compared ambient erythemal doses calculated for various time intervals with average minimal erythemal doses (MEDs) of the Caucasian Fitzpatrick skin phototypes I-IV to assess the potential for skin damage arising from sun exposure on days with low UVI values. The most common months for the occurrence of days with low UVI values in our dataset were January and December, February and November, and March and October for UVI 0, 1 and 2, respectively. Our results indicate that on days with a UVI value of 0, risk of deterministic radiation injury (solar erythema) is negligible. Conversely, the above-mentioned health message appears misleading when melano-compromised individuals spend several hours outdoors on days with a UVI value of 2, as median doses exceed the MEDs of Fitzpatrick skin types I and II after an exposure duration of only 2h around solar noon. Under very rare specific circumstances, MEDs of those two most sensitive skin types can also be exceeded even on days with UVI 1. Hence, two aspects of current public health messages may need reconsideration: on the one hand, the health message related to an ‘innocuous level’ of the UVI and, on the other hand, a possible adaption of UVI-related health messages to different skin types.
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