Table 2.

Potential yield of vitamin D₃ formed in skin from a single exposure to UV sunlight

	5 % skina exposed (μg)	10 % skina exposed (μg)	20 % skina exposed (μg)
7-DHCb at 0·5 μg/cm2	458	917	1834
Vitamin D3 produced at 10 % conversion	46	92	183
Vitamin D3 produced at 20 % conversion	92	183	367

The yield of vitamin D₃ in skin, in response to exposure to UV light from the sun, depends not only on the number of minutes of a single period of exposure, but also on many other variables, including the seasonal intensity of solar UVB radiation, absorption of UVB radiation by skin pigmentation, the age of the irradiated individual and the proportion of pre-vitamin D₃ produced that is then converted by prolonged irradiation, to other non-biologically active products. Experiments with human skin, irradiated in vitro, have found up to 35 % of total 7-DHC that can be converted to pre-vitamin D₃ in a single exposure session⁽¹⁶⁾.

^aBased on the calculated total skin area of an adult male of 18 229 cm²⁽¹⁵⁾.

^bLowest concentration of 7-DHC in skin, which ranges from 0·5–1·3 μg/cm²⁽¹²⁾.