How Much Skin Should You Expose?

Q. One thing that I have not seen in the discussions of vitamin D relates to sun exposure. You have said that people who get 10 to 15 minutes of sun exposure at one time can make enough vitamin D in their skin.
On what percent of the body is it necessary to get that exposure in order to convert the sun into 10,000 IUs/day? Just the face? The hands? Bare butt naked? It’s a very obvious omission from the data that needs to be explained.
A. The exact amount of sun exposure needed varies according to the time of day, the season of the year, the latitude and the color of the individual’s skin. (Darker skin needs more time in the sun to make the same amount of vitamin D.)
The usual average of 10 to 15 minutes a few days each week applies to the face and hands of a light-skinned person in the summer. Winter exposure (even if one could tolerate it) in places like Boston or Milwaukee won’t provide enough sun for the skin to make vitamin D. An oral supplement will be needed.