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.