If a mist elf wears a metal item + ring(s) of regeneration in a dungeon it is safe since every turn you lose hp and immediately gain it back.
But if you go to the world map and move, you will first get hit a couple of times by the metal item, and only then regeneration kicks in and gives you back HP a couple of times.

Dungeon: <get hit><regen><end turn> ; <get hit><regen><end turn> etc. etc.
World map: <get hit> x8 <regen> x8 <end turn>;

When low on HP this small difference can get you killed on the world map even if you would be 100% safe in a dungeon.

I believe the dungeon sequence should be in effect in both cases.

Also haven't checked this for other kinds of damage over time...if there are any :)