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Comparing UV light ELVs with natural sunlight exposure

A useful perspective is achieved by comparing the UV light exposure limit values with examples of natural outdoor UV light exposure. Comparison of the UV-A light radiant exposure limit value (HUV-A max) of 10,000 J/m2 for the unprotected eye with natural sunlight exposure.

The Health Protection Agency (HPA) continuously monitor solar UV light levels at six sites in the UK.  These are located approximately every 2o of latitude from 50oN to 60oN.  Information from these sites is presented in terms of the UV index (UVI) to the UK Meteorological Office.

Using the Heath Protection Agency’s data, a good comparison example is obtained by calculating the average UV-A light irradiance values (EUV-A) for natural sunlight at noon during the UK summer and winter periods.

The maximum permissible exposure times (tUV-A max) in seconds to UV-A light incident upon the unprotected eye are calculated by dividing the maximum permissible UV-A light radiant exposure value (HUV-A max) of 10,000 J/m2 by the average UV-A light irradiance values (EUV-A in W/m2).

This data is presented below and shows the average UV-A light irradiance values (EUV-A) for natural sunlight at noon in UK summer and winter periods, with corresponding maximum permissible exposure times (tUV-A max).

Natural noon sunlight – UK winterNatural noon sunlight – UK summer
Average UV-A light Irradiance
(E UV-A)
Maximum permissible exposure time for the unprotected eye
(tUV-A max)
Average UV-A light Irradiance
(E UV-A)
Maximum permissible exposure time for the unprotected eye
(tUV-A max)
0.77 mW/cm221.6 minutes2.8 mW/cm25.95 minutes

Comparison of the UV light effective radiant exposure limit value (Heff max) of 30 J/m2eff for the unprotected skin and eye with natural sunlight exposure.

The exposure limit can be exceeded by exposure to the theoretical noon UV light effective irradiance values from the overhead sun at 40o to 70o latitude for summer months within 30 minutes.

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