The reaction is as follows: 7-dehydrocholesterol --(UVB @ 296nm)--> previtamin-D --(thermal isomerization)--> cholecalciferol The first step reaches equilibri

The reaction is as follows: 7-dehydrocholesterol --(UVB @ 296nm)--> previtamin-D --(thermal isomerization)--> cholecalciferol The first step reaches equilibrium after about 15 minutes in Fitzpatrick type 1 skin in UV index 3-5 (moderate), and about 2 hours in Fitzpatrick type 6. The total amount produced is dependent on the surface area of exposed skin, and the previtamin isomerizes slowly, over about 12 days. Time to equilibrium is about half the time to acquire sunburn at a UV index high enough to catalyze the previtamin-D reaction. At UV index 6-7 (high), time to burn is about the same as the time to reach previtamin-D equilibrium. At UV index 8 or higher (very high), you will burn faster than you make previtamin-D, and should maybe avoid being exposed during the midday hours. At UV index 11 or higher (extreme), if you are Fitzpatrick type 1, you will carbonize to a burnt matchstick the instant you step out of shadow, like a cartoon vampire, and you should not only not be outside, but should also be huddling in a dark basement waiting on the tyranny of the sun to pass. Thanks to the equilibrium in the skin, the maximum absorbed dose from sunlight is the equivalent of about 20000 IU per day. High vitamin-D intake also requires increased vitamin-K consumption to reduce calcification of soft tissues.