As the summer season approaches

To prevent Vitamin D deficiency it is recommended by Dr. Holick to walk with your face, hands and arms exposed to the sun two or three times a week for about half the time it would take to get sunburned-between ten and twenty minutes depending on your skin tone, the season, and the distance from the equator. Most importantly, Dr. Holick recommends this exposure to the sun be without sunscreen, as sunscreen interferes with the production of vitamin D. Sunscreen with an SPF (sun protection factor) of 8 allows only 5 percent of normal production of the vitamin, while with an SPF of 30 virtually none is produced.
Holick -2004