Being numb to pain is as good as being immune to it. It's like when you go to the dentist. You get your mouth numb for the drill so it doesn't hurt.
Thats when a specific area is injected or has anaesthetic cream applied to it. A gas would affect the entire body.
Immune means you wouldnt be affected by it ever, after the anaesthetic wears off you would still feel the pain, you are only "numb" to it during the procedure.
Its not just his face that was injured, he also has a scar running the entire length of his spine.
http://i.imgur.com/ojqSQ.jpg
Watching the scene again shows that what was said is he was left in perpetual agony and the mask holds the pain at bay, it doesn't specify an area or say just his face was in pain.
http://www.youtube.com/watch?v=NyXEr8lS2dw
Finally, a link to an interview with Nolan himself who doesn't specify it was just his face, but merely says he is a man ravaged by pain.
www.flicksandbits.com/2012/05/17/ch...m-hardys-bane-in-the-dark-knight-rises/25919/