And yet the RN are OK with beards. Not much H2S and CN but plenty of other hazards around plus a need for flash protection. Odd that.
Different risks though and different acceptable levels of risk? On an oil rig you’ve got an expectation of going home safe. Army/navy/air force there are other dangers.
Respirators to protect against fire probably dont need to seal particulalry well, just provide fresh air to breathe, whereas a respirator to wear in the event of a leak of toxic gas needs to seal perfectly because H2S and CN in particular will kill at ppm concentrations.
And a ship is effectively indoors, shut the door and your safe from whatever gas is on the other side. Same way my beard is/was perfectly acceptable working on most sites and offices in Teesside but wouldn’t have been allowed out of the office building at Billingham.
As for discrimination. No different to similar policies which effectively ban wheelchair users, the blind or fat people. Its not discrimination, its writing job descriptions which say you must be able to climb ladders/stairs, drive a car and get through a helicopter window.