Mmm
I think the grease comes from your skin unless the individual has over indulged with the inflight meal! Grease + Oxygen + Ignition = explosion!
For full fuel tank entry engineers require a mask fitment test carried out by a superb piece of kit from Sabre. Ive been told our fire fighters use the same. It measures leak rates, negative pressures and positive pressures from all head angles. I have a beard and pass this Sabre test every time.
I can ensure that the mask we use is inferior to the standard of mask that is located in the flight deck.
I'm sure a few dodgy masks make into the crew's cases for the night stop though lol!
K Regards