Surely those big handle bar moustaches some of the QF chappies have, would make sealing of an oxygen mask far more difficult than a neatly trimmed beard, in fact you may not even get the mask over some of them!
I personally think it's a croc, the quick donning masks flight crew wear have a positive pressure, air will only leak out, not in, especially considering the pressure will be higher inside the mask.
What do we need a tight seal for anyway, unless you are planning a little underwater expedition!
