Comparison of UV light exposure limit values with natural sunlight

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 eyes with natural sunlight

The Public Health England (PHE) 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.

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 eyes 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 in the table 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 winter UK summer
Average UV-A light irradiance value EUV-A Maximum permissible exposure time (unprotected eyes) tUV-A max Average UV-A light irradiance value EUV-A Maximum permissible exposure time (unprotected eyes) tUV-A max
0.77 mW/cm2 21.6 minutes 2.8 mW/cm2 5.95 minutes

Comparing a specific UV light exposure scenario with natural UV-A light exposure

This can provide a very useful perspective and is particularly useful in circumstances involving deliberate UV-A light exposure of specific persons. A good example is a routine inspection of detainees under UV blacklight, for SmartWater detection in police custody suites.

A typical installation for this application is shown by the photograph in the table. Detainees are exposed for 30 seconds at a minimum distance of 1 m from the UV-A light sources. This equates to only a fraction of the ELVs and equivalent to approximately 6 seconds exposure to winter sun and 2 seconds exposure to summer sun at around noon in the UK.

Minimum distance UV-A light irradiance value EUV-A Maximum permissible exposure time tUV-A max (without eyes protection)
1,000 mm 0.14 mW/cm2 2 hours

Comparison of the UV light effective radiant exposure limit value (Heff max) of 30 J/m2 eff for the unprotected skin and eyes with natural sunlight

This exposure limit can be exceeded by exposure to the theoretical noon UV light effective irradiance values from the overhead sun at 0o to 40o latitude for most months throughout the year within 15 minutes.

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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