Originally Posted by sailingfun

It does not prevent O2 flow but does reduce overall efficiency of the mask and the ability to seal out toxic fumes. The question is how much of a reduction is acceptable. That amount varies between facial types and mask design. In professions that deal with highly toxic chemicals you won’t find beards. Most people stating it’s a non issue quote one very limited Air Canada study using one type of mask. There are lots of other studies out there.

a. caused a
respirators. average
Several investigators had reported that the presence of beards decrement in the efficiency on half masks, full masks, and
-. A Department of Navy study (No. NADC-722110CS) reported an '
inboard leakage of 16 to 67 percent for military-type crew oxygen
masks when tested with subjects wearing beards to altitudes of 18,000 feet. Civilian crew oxygen masks vary considerably from these types of masks
because of differences in basic mask designs, suspension systems,.,and controlling regulators. Consequently, CAMI conducted'research to determine if the problems noted in these reports would also be present when civil aviation oxygen equipment was tested.
b. During the CAMI research, three popular TSO-approved 'crewmember oxygen masks equipped with mask-mounted regulators were tested to determine if a decrement in performance would occur as a result of the presence of
facial hair. The data resulting from these tests indicated
does occur when facial hair is present along
that decrement the sealing
oxygen equipment manufacturers, concluded that respirator/oxygen
referred to as "demand") masks cannot be donned rapidly and do not seal over beards or heavy facial hair. This lack of a seal could result in a reduced amount of oxygen in the mask and the entry of smoke or toxic fumes that could result in reduced crewmember capability and performance.
in performance surface of
oxygen masks. This decrement present, the type mask worn,
is proportional suspension
to the amount system
crew hair
of facial
associated with the mask, and the exercise level to which the individual is subjected. This study, as well as studies conducted by the military and
the
(sometimes

Originally Posted by RemoveB4flght

BTW the rationale behind the beard thing is religious, and offending the deity > O2 mask seal