# Erythermal Solar Irradiance, UVER, and UV Index from Ground-Based Data in Central Spain
## 3 Results
### UVI, Cumulative Doses, and Risks  
UVI reports the instantaneous sunburning potential of solar UVER irradiance and supplies information about the risks of sun exposure [10]. In addition, sun protection studies are given in different public actions. These state that sun protection warning is not necessary if the UVI value is below two. Different authors explain how sun exposure damage depends on daily UVI and cumulative daily dose SEDs and not only on UVI [33]. In this section, the relation between maximum daily UVI and cumulative daily dose in SEDs is studied through UVER measured data to explain the above recommendation concerning sun exposure dependence.  
Regarding the results, Figure 6a shows the experimental yearly evolution of maximum daily UVI and daily dose in SEDs measured in Valladolid, Spain, as a function of the day of the year. It can be seen that the maximum daily UVI in summer exceeds 12 (Figure 6a) and that the maximum daily dose can exceed 60 SED (Figure 6b) [33; 34]. However, the number of daily SED values below the 2.5 SED threshold is very small.  
There is therefore sufficient UVER solar radiation on summer days to cause 24 MEDs of erytomal doses for skin type II (due to 60 J/m\({}^{2}\) being equivalent to 24 MEDs, or 1 J/m\({}^{2}\) to 2.5 MEDs). This means that sunscreen should have a solar protection factor (SPF) of above 24. Over the winter months, the maximum UVI for clear days is one or two, while the daily dose is between 2.5 and 5 SEDs (see Figure 6).  
Figure 7 shows the erytomal irradiance daily dose in SEDs as a function of the maximum UVI value for each day. The data envelope curve corresponds to days with clear skies. Figure 7 shows that there are very few days whose daily doses are below 2.5 SED. In addition, between the maximum UVI below two and the daily dose above 2.5 SED, there are many days where the maximum UVI is below two but the daily dose is above 2.5 SED. Figure 7 shows that the damage threshold for fair skin is exceeded almost every day, including those for which maximum UVI is in the range 0.5 to 3 [23; 24]. It can be seen that there are many days when UVI is below 3.0, but that the daily dose is above 10 SEDs and that on a few occasions it even reaches 14 SEDs. Figure 7 shows a blue point, the coordinates for which reflect a UVI of 3.0, and a daily dose of 17.5 SEDs. Other places have reported similar results, with daily doses at mid-latitude sites surpassing 10 SEDs when the maximum UVI is below 3.0 and when visible damage to fair-skinned people can occur in little over an hour [35].  
A review is required of the information provided concerning sunburn in mid-latitude areas, when UVI \(<\) 3 and sunscreen protection did not recommended. Although the issue might not be as serious in winter months and at high latitudes, such as Northern Europe [36] where maximum UVI is often below 1 and where the daily dose is below 2.5 SED on many winter days. However, similar issues will arise in seasons when maximum UVI values approach UVI \(=\) 3.0 [35; 37].  
In light of the results, and following various authors' statements [35; 36; 37] to the effect that \"no protection is required for UVI\(<\) 3\", public authority recommendations need to be removed or appropriately amended as it is known that skin damage is a function of two variables: UVI and time of exposure, which are related to the cumulative daily dose. Specific information about not needing to use protection should only be given for days when the cumulative daily total UV dose is below the damage threshold; namely, 2.5 SED for skin type II, as can be seen in Figure 7 [35].  
Figure 6: (**a**) Experimental yearly evolution of maximum daily UVI (**b**) and daily dose in SEDs, measured in Valladolid, Spain.  
Figure 7: Daily dose of erytomal UV as a function of maximum UVI measured in Valladolid, Spain. The blue dot shows the mean SED for UVI = 3, calculated for clear days at the measurement station.