I am not emotionally invested in this in any way, and frankly couldn’t care less about others’ facial hair choices. But I suspect that LJ is on to something with the HSE reference. This from
HSG53:
82 The wearer needs to be clean-shaven around the face seal to achieve an effective fit when using tight-fitting face pieces. Training is a good opportunity to make employees aware of this. If workers have beards, or are unable to be clean-shaven, a tight-fitting device will not be suitable so an appropriate loose-fitting device should be chosen.
From experience with other topics on which the HSE has published guidance (for instance, fatigue management) I know that a very compelling case needs to be made before Duty Holders will go against a HSE ‘should’ (which is similar in meaning and legal effect to a MAA ‘should’). The quote above uses ‘needs’ and ‘will not’ which are unequivocal terms and leave even less (...no) room for manoeuvre. The only ‘should’ in that quote is one that probably excuses religious beards, on the basis that a loose-fitting device is impracticable.
Now, you can say the HSE publication is wrong, or that the Duty Holder can take a risk (because oxygen masks only act as ‘filtering devices’ on the very rare occasions when smoke and fumes occur), or that a case could be made that slight overpressure eliminates any risk, but to what benefit? That would be the question tested in any court case and that’s why these things can be difficult to go against.