The gas can be thin at the edges, so you have a bit of time to react heading down ramps or hills, etc. In one spot in Broken Valley one of Bellegar's shrine areas appeared to be relatively free from gas, but upon jumping down I found It wasn't and had to quickly jump back up.
However, in most places the fog is thick enough that it is pretty much an instant kill; you can not keep healing yourself to actually make it anywhere. In Broken Valley there was one small walled platform that I checked out, where the top of the walls were above the gas, but jumping down to the floor 8 or 10 feet below was deadly, with no chance to heal or jump back up.
There is a hidden cave on sentinel island that you can reach by swimming. Trying to see if it was possible to get to it after getting the battle tower, I maxed Regeneration, put healing auras on all my jewelry, used a healing potion, etc, and managed to drop though the poison cloud and hit the water, but didn't have a chance to even move before dying, let alone swim to the entrance.
When flying around the island, though, the poison cloud thins out as you move away, and the dragon has more a bit more tolerance for the gas (though it is still deadly if you fly too low near the island).
