Having had the cockpit fill with smoke/fumes on rotation recently, I can confirm that an Eros mask on 100% is pretty effective at keeping the nasties out, although I don't have any face furniture. I was aware that if I needed to clear the mask out I could give a little twiddle and/or push on the knob to remove any unwanted fumes/aerosols.
I agree that a better quiescent state for the mask would be to have "EMER" selected as you can always unselect it after the event but our checklist (777) says to have it on 100%...
