Sunburn can be controlled by observing the accumulated UV exposure. This measure can be described with the "UV index hour" unit.

Some explanation for the MED - UVIh conversion:

The general MED (minimal erytherma dose) values for the different skin types (Fitzpatric skin type scale) are the following:

Type I - 200 J / m2
Type II - 250 J / m2
Type III - 350 J / m2
Type IV - 450 J / m2
Type V - 600 J / m2
Type VI - 1000 J / m2

Conversion to UVIh:
J / m2 = Ws / m2
1 UVI = 25 mW / m2
1 UVIh = 25 mW / m2 * 3600s = 90 Ws / m2
UVIh = MED / 90