Vitamin D deficiency is quite common, and a growing list of diseases and conditions are being linked with it. Regular sun exposure, without sunscreen, causes your skin to produce vitamin D naturally. But how much sun do you need?
You've probably seen some vague guidelines, recommending "a few minutes every day." But these recommendations are far too general to be useful. The amount of sun you need to meet your vitamin D requirements varies hugely depending on your location, your skin type, the time of year, the time of day, and even the atmospheric conditions.
The vitamin D/UV calculator
Scientists at the Norwegian Institute for Air Research have devised a calculator that will take all those factors into consideration and estimate how many minutes of exposure you need for your skin to produce 25 mcg (the equivalent of 1,000 International Units) of vitamin D.
It's not the most user-friendly interface and it is very easy to enter the wrong information. But once you get past the technicalities, it's very interesting to see how much the answers change when you vary the input.
You'll need to determine your latitude and longitude. You can find this information on Google Earth. The program reads all latitudes as N and all longitudes as E. If your latitude is 39 S, enter -39. If your longitude is 76 W, enter -76. You’ll also need to enter the time of day you are going out in the sun, expressed as UTC (Greenwich Mean Time). Here is a converter that will convert local time into UTC. The calculator uses a 24 hour clock, so hours from 1 PM to midnight are expressed as 13 to 24.
The calculator also wants to know the thickness of the ozone layer. I suggest just setting this one to medium. Then, find your altitude. Google Earth can tell you this, but the easiest way may be to simply google "altitude of [your town]". Remember to convert it to kilometers. One kilometer is about 3300 feet.
Be sure to click the radio button next to the entries. They are often not automatically selected when you fill in the values.
Keep in mind that the exposure times given are considered enough to maintain healthy vitamin D status. If you are starting out with a vitamin D deficiency, you might need more.