[lch]

The difference is in how much poison the unit initially obtains or accumulates. Depending on that, it determines how much poison damage the unit gets per turn. With Death poison or strong poison, this can be as high as 5 HP or more per turn for the first couple of turns, I didn't check their damage. With Foul Vapors, this will probably be no more than 1-2 damage per turn, as has been remarked earlier. That could kill 10 HP units, but they probably have a good chance of making it out of this alive.