Oh, i do a lot of that. The green gas can be quite prolific though. At one point i mined out about 50 blocks above me and it was still thick. I generally try to move down a couple of blocks and keep going forward at that point. It can be dealt with, that's not the problem. But it certainly makes you work harder at mining, and if you get sloppy...bad things happen.A good way of dealing with gasses is to simply dig holes in the floor or ceiling, depending on the density of the gas. It just sinks or floats into the hole you made, then if you replace the blocks it destroys the gas just like when placing a block in water or lava.
When i move down to the lower levels is where i slow way, way down. The explosions are just so devastating.