The chests do fire damage when you open them. Wear armour with close to 70 fire resist and you will survive.
Be aware that level 2 chests (those with level 2 maps in them, that come from paragons of creatures that normally give level 1 maps) are bugged.
On occasion, level 2 chests (both the paragon chests, and ones from actually digging the level 2 maps) will do several times more damage than they are supposed to. Normally, they'll do well under 50 points of damage through 70 fire resist.
I don't die opening chests EXCEPT the occasional bugged level 2.
In the case of the bugged level 2s, I have taken as much as 159 points of damage THROUGH 70s resist armor (and given that I only had a little over 120 HP at the time, it was fatal)
And, there's no way of telling beforehand which ones are bugged.