So the wiki says steam is harmful in the new version, and we have no idea about the lethality of magma mist.
Tested in 0.31.05.
First test: submitting a rabbit to a constant source of steam, aka a magma man in a water flow in the Arena.
I make an 1-tile island in the rivers of the Arena, put a rabbit on it, and then surround it with magma men. The magma men plunge inside the river and start to boil it.
Multiple clouds of steam enter the rabbit's tile, no damage is observed. Gonna now test the magma mist, and see if prolonged exposure to steam hurts the rabbit.
As for the magma mist, I will drop a 4x4x1 block of rock in the magma pools, creating large amounts of magma mist, wait for the cave-in dust to disappear, and create fliers in the magma mist caves.
Cave-in happens. A giant cloud of dust of various inorganic material happens (no soil though, but yes, there's even slade dust, copper dust, gem dust and rock dust, as well as bituminous copper dust). Nice bug right there.
After a while, the mineral cloud disappears, but not the lava mist. I therefore spawn 50 masked lovebird men inside (mostly) the lava mist column.
I pause, then immediately after unpause. 15 are already !!Masked Lovebird Men!!
After a while, more than the half are burning.
And after the cloud dissipates, only nine have survived. Seems these didn't get touched by the magma mist.
I go and see the rabbit, which has nearly two pages of "being caught in a burst of steam". No damage at all.
Conclusion : Steam still does nothing. Magma mist however...