Q: Regarding sunshine and vitamin D… just how much skin needs to be exposed in the 20 minutes a week for the sun’s rays to give you the amount of vitamin D needed?
A: Your question is a very good one indeed. While uncovered skin will absorb the maximum amount of ultraviolet rays, research shows that they pass through some fabrics such as open-weave fabrics, lightweight knits, and nylon stockings. Also, t-shirts worn while swimming allow ultraviolet rays to pass through, and ultraviolet rays even penetrate water, but lose half their intensity.
Fortunately, these rays are very stubborn: they are present on hazy days and bounce off bright surfaces, such as snow, pavement or sand.
So the main thing is to spend a little time outdoors and have fun in the sun.