We're not going to talk about sunblock today. We're going to talk about vitamin D and the fact that your skin can magically make vitamin D from exposure to the sun.
It is said that a mere 15 minutes of exposure is enough time for your body to magically provide you with the necessary daily dose of vitamin D.
In truth however, it depends.
Not only does it depend on which part of the planet you are living, but also what time of year you decide to soak up some rays.
Above or Below The LineSome clever folks who live north of me have done a bit on research and study on the skins ability to synthesize vitamin D from sun exposure and have published their findings.
I've linked to them below, but here are the headlines:
If you're living above the 51st parallel north (or south), you aren't getting a blessed bit of vitamin D from sun exposure at certain times of the year.
If you're living above the 70th parallel north, feel free to enjoy the sun for about 7 months. As for the remaining 5 months, any exposure to the sun isn't going to give you the teensiest bit of vitamin D benefits.
Not sure where you are living as to the north and south parallels? Check the map below. I made it just for you (and me, because I wasn't sure either).
Check the links below if you want access to an online vitamin-D skin synthesis calculator to see how well you are covered in your area of the planet.
Photochemistry and Photobiology (by 3 clever Nordic folks)
More About Vitamin D synthesis in human skin
Duration of vitamin D synthesis in human skin
Computing vitamin D synthesis in human skin
Wikipedia on human skin