Polymorphous Light Eruption!
Over the last few years I have been getting what I thought was a heat rash after spending a lot of time in the sun at Lake Powell, or Hawaii, or wherever.  Turns out, I went California and visited my Aunt Jeane Anne and she said she has the same thing.  So I went to the Doctor and he said basically that I am allergic to the SUN!  Basically my skin thinks that UV rays are the same as something like poison ivy, and it creates a rash to get rid of it.  It is a genetic chemical reaction! Crazy huh?  The Doctor says it can appear when people are in their twenties, and it NEVER goes away.  His solution: wear 75 spf, stay out of the sun or wear long sleeves ... awesome.  I have been prescribed to the life of a hermit.  The rash doesn't really ever get that bad, unless I am on a beach vacation, so maybe I can find a happy compromise.  I guess I will never really be that tan again, not that I ever was but don't laugh if I am pasty white in July next year.

Comments
We like the Neutragena SPF 75 and 80 down here.
I first started getting at 18 or so. Major bummer...but easily managed.