Taken from the Wiki, Bane's mask explained:
Bane was attacked and severely beaten by the other inmates. The penitentiary's resident doctor attempted to heal his injuries, but despite his efforts Bane ended up in a state of constant, excruciating pain. The mask, crafted following Bane's release, provides him with a constant stream of anesthetic gas to numb the pain and allow him to function.
Fucking stupid.