I guess his brain is somehow connected to his helmet - and he cannot survive without it. The breath mask that makes his breath so sinister may be a pollution filter as well - these are educated guesses, so if anyone is a SW afficionado, please correct me if my guesses are wrong.
Do i really? i've never tried... arrggh.... splutter... cough... cough....
The official answer is that the helmet acts as an air filter to stop poisons entering my head via airway or through my badly damaged scar tissue! (My brain is NOT attached to it as you saw in Return of the Jedi where i took it off to talk to Luke, the ungrateful little b@$tard that he is). For the unofficial answer you'll have to ask Mrs Vader.......
I'd say that it is quite simple. In Empire we see him with his helmet off (from the back) while in communication with the Emperor, thereby suggesting that he can have his helmet off and live. However, in Return of the Jedi he is dying and says that he wants to take the mask off to look at Luke through his own eyes. It is Luke that suggests taking the mask off will kill him and, really, how would Luke know anyway?