Table 1. Exposure times needed to achieve vitamin D concentrations of 200, 400, and 600 IU, depending on the UV index.

UV index (1 = low, 12 = high)
	1	3	6	12
Exposure time (min) for the production of:
200 IU vitamin D	46	15	8	4
400 IU vitamin D	93	31	16	8
600 IU vitamin D	140	47	24	12
Skin type II: minimal erythema doses per hour (MED/h)	0.42	1.26	2.52	5.04

For a person with skin type II, the exposure of 6% of the skin surface to one minimal erythema dose (MED = 210 J/m²) results in the production of 600 IU of vitamin D (e19). One unit on the UV index corresponds to an irradiation intensity of 0.42 MED/h. These figures were used to generate the table above. The minimal erythema doses for less UV-sensitive skin types (types III and IV) range from 350 to 450 J/m². The exposure times for skin types III and IV would be easy to calculate from this information and from the measured figure of 600 IU Vit D per 210 J/m² for skin type II if there were a simple, linear relationship between the exposure time and vitamin D production. Recent studies have revealed, however, that vitamin D production involves as many as 9 different (photo-)isomerization reactions; thus, simple, linear extrapolation of data from skin type II to other skin types would be a misleading oversimplification. It remains true, of course, that the exposure times for skin types III and IV are longer than those for skin type II that are listed in the table.