Why wait to shower after UV exposure?

I was trying to find some information for a friend about Vitamin D, and I came across an article from Natural News that I thought was very interesting.  So why wait to take a shower after UV exposure?  We have that question or "How long should I wait to take a shower after tanning?" asked here a lot.  According to Danna Norek, author of the article in Natural News,  to help your body absorb more vitamin D you need to wait as many hours as possible because "after UV exposure, your skin produces a powdery substance that is then absorbed into the skin and converted to vitamin D by the body. If you wash this off too early, you interfere with the amount of vitamin D made, and hence do not get the full benefit."  She also included tanning indoors but just don't burn. :)  That is our motto here!  http://www.naturalnews.com/030500_vitamin_D_absorption.html