I read somewhere that Adolf Hitler's short trimmed moustache was the style worn in the trenches during the Great War to ensure a good seal around a gas mask, whilst still looking German! By wearing it, he was overtly reminding everybody "I fought in the trenches too".
Clearly the Royal Navy still wear them
I tried growing a moustache once, itched like hell and made me sneeze so I shaved it off. My wife assures me that a beard would be a very bad thing and likely to have a detremental effect on my love life. That worries me far more than the very rare occasions I might ever wear an oxygen mask.
Just found on an RAF website:
Muslim men are permitted to wear short, neatly trimmed beards. However, for occupational or operational reasons, where a hazard clearly exists, Muslim men may be required to modify or remove facial hair to such an extent as to enable the correct wearing of breathing apparatus, oxygen masks or respirators. Notwithstanding this, Muslim men will not be required to shave their beards in order to perform Nuclear, Biological and Chemical training drills.
G