The term "erythema" refers to the reddening of the skin due to sunburn. For UV-induced erythema, the action spectrum adopted by most international organizations is the CIE, International Commission on Illumination's action spectrum (E), using the method described by McKinlay and Diffey (1987a, b). The erythemal action spectrum is specified over three spectral ranges as: (1) W(X) = 1 for 250 nm < X < 298 nm, (2) W(X) = 100 094(298 "2) for 298 nm < X < 328 nm, and (3) W(X) = 100015(139 "2) for 328 nm < X < 400 nm. The UV-I itself is an irradiance scale computed by multiplying the CIE irradiance in watts m by 40. For a fairly wide range of atmospheric conditions, the CIE weighted irradiance changes by approximately 1.2% for a change of 1.0% in the ozone value. Thus the clear sky value at sea level in the tropics would normally be in the range 10 -12 (250 nm -
10 being an exceptionally high value for northern mid-latitudes. This scale has been adopted by the WMO and the World Health Organization (WHO), and is in use in a number of other countries (Environment Canada, 2008). Ultraviolet intensity is also described in terms of UV-I ranges running from low values (0 - 2) to medium (3 - 5), high (6 - 7), very high (8 -10) and extreme (11 + ). Other irradiance integrals exist which describe other physical and biological effects of UV radiation. However, this one has become the one used most often (Environment Canada, 2008).
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