The question specifies
smoke in the cockpit and flight crew protective breathing equipment makes use
of the crew’s normal oxygen masks with the addition of smoke goggles.
The masks form a seal to
the face so the ingress of smoke is not a problem.
The normal oxygen supply
is a diluter system which cannot be selected when smoke is present so 100%
oxygen is selected.
Smoke hoods for use by
cabin crew may require a positive pressure to prevent the ingress of smoke.