It has long been known that many diseases are more common in northern climes than southern. This was a mystery until recently medical researchers found that Vitamin-D is much more important than was once believed. It is vital to our immune system and they have found that the average person doesn't get enough Vitamin-D. Vit-D is a particular vitamin because it is not absorbed well from food or vitamin pills. The best source is produced by our skin from sunlight. This is not to say you should bake for hours in the sun. Like everything- sunning can be overdone- but it doesn't take a lot of sun exposure- in fact- ironically- fair skin people need less sun exposure to produce their Vit-D. This was part of the evolutionary process as man migrated from southern climes like Africa to northern climes like Europe- he lost skin melanin to compensate for reduced sunlight to maintain Vit-D production. Nature knew how important Vit-D is!
So you should never be in the sun to the point of erythema- that's way more than is necessary for Vit-D. So as you note, the ASA recently changed their name to the American Association for Nude Recreation (http://aanr.com
) in order to emphasize sports and exercise over baking in the sun. With complete skin exposure you get all the Vit-D you need during a game of tennis or volleyball without any part getting too much.