Ed Friedman writes:

From my latest reading it appears that this is not necessarily the
case.  What is dangerous is skin burn from sunlight.  Tanning and
exposure to the sun do not seem to be the culprits, but burning the
skin.  So the American approach is, of course, to slather sunscreen
on to avoid the whole issue.  One consequence is that people don't
get enough Vitamin D.  Brief exposures to sunlight produce far
more useable Vitamin D than most supplements.  There is a
contrarian perspective on exposure to the sun suggesting that
the continuous use of sunscreen not only lowers Vitamin D
levels, but actually may backfire.  People do not tan and gain
resistance to sunburning.  Instead they use sunscreen and then
when they forget or are out too long they burn.  If they had
simply helped their skin adapt by moderate exposure without
burning then they would avoid those kinds of burns and
skin cancer.

OCL