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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