3.  Definition of the UV Index and its physical explanation

UV radiation and action spectra
An action spectrum describes the relative effectiveness of UV radiation at a particular wavelength in producing a particular biological response. The biological response may refer to various detrimental effects on biological subjects including humans, animals or plants. An action spectrum for a given biological effect is used as a wavelength-dependent weighting factor to the spectral UV irradiance (280 to 400nm) and then integrated over wavelength to find the actual biologically effective irradiance (in W/m2). The effective UV dose (in J/m2) for a particular exposure period is found by summing (integrating) the effective irradiance over the exposure period. The most important responses for common use are the erythemal, DNA absorption and non-melanoma skin cancer action spectra.

Minimal Erythemal Dose 
As sunburn is a frequent detrimental effect on human skin the CIE Erythemal action spectrum is recommended for use in assessing the skin-damaging effect of UV radiation. The ”Minimal Erythemal Dose” (MED) is used to describe the erythemal potential of UV radiation and 1 MED is defined as the effective UV dose that causes a perceptible reddening of previously unexposed human skin. However, because human individuals are not equally sensitive to UV radiation due to different self-protection abilities of their skin (pigmentation), 1 MED varies among the European population with a range of between 200 and 500 J/m2. If national studies of the erythemal sensitivity of the population are not available the values of MEDs for different skin types according to DIN-5050 shown in Table 2 may be consulted.

The UV Index - a UV parameter for the public
Originally the UV Index was formulated independently in several countries and used in programmes for public information about UV radiation. Its definition was later standardised and published as a joint recommendation by the World Health Organization (WHO), the World Meteorological Organization (WMO), the United Nations Environment Programme (UNEP) and the International Commission on Non-Ionizing Radiation (ICNIRP) (see Appendix E). The UV Index is recommended as a vehicle to raise public awareness about the potential detrimental effects on health from solar UV exposure and to alert people of the need to adopt protective measures. If cloud cover and other relevant environmental variables are  accounted  for  when calculating  the UV Index,  the modifying  factors that are used  in  the calculation should be stated. Although the UV Index is defined for an exposed horizontal surface the exposure conditions for an inclined surface may be more relevant to human exposure. If the UV Index is referring to an inclined surface it must be stated.

UV Index forecasting 
Operational UV Index forecasting has already been implemented in many countries (see Appendix C and D). The forecast methods vary from simple statistical methods used for local areas to more complicated methods with global coverage and with forecast times from a few hours to several days, either for clear sky or all sky conditions. A general forecast approach is described in the diagram shown in Figure 1.  The accuracy of UV forecasts is limited mainly by the amount and quality of the input data. In the future, a large-scale assimilation of ground-based and satellite observations of ozone, aerosol and clouds may considerably improve the accuracy.


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