On 26-May-2005, john smith wrote:

The partial pressure of O2 at FL180 is 50% that of sea level.
As you go higher, the rate of partial pressure change decreases more
rapidly, so that by FL270 it is into single digits.
A pressurized mask is necessary to force the air into your lungs and
into your blood.

Actually, you nailed the main issue -- oxygen partial pressure -- but missed
the precise reason for the pressure mask.

As altitude increases, partial pressure of O2 decreases, as you correctly
state. The O2 PP at sea level is way more than we need, and healthy
individuals can tolerate the air at up to about 14000 ft without much loss
of mental capacity (physical capacity, including visual acuity, is another
matter.) Above that (actually, above 12500 ft), in an unpressurized
airplane, we increase the PP of the O2 we breath by adding supplemental
oxygen. The cannulas and/or masks typically used in light aircraft mix pure
O2 from the bottle with atmospheric air at a ratio that can be somewhat
adjusted (by the flow adjustment) to result in roughly the desired O2 PP.
But these systems are not designed to deliver anything close to pure O2 to
the user. The maximum O2 concentration they CAN deliver will provide
sufficient O2 PP at about 18000 ft for cannulas and about 26000 ft (if I
recall correctly) for masks. Above that, you need a system that can deliver
higher O2 concentrations, up to and including pure O2, which is where the
pressure mask comes in. What it does is provide O2 at slight positive
pressure relative to the outside air to prevent uncontrolled mixing in the
mask. Such masks also cover both nose and mouth to prevent leakage/mixing
that way. It doesn't really "force" the O2 into the user's lungs, nor does
it need to.

Above about 35000 ft even pure O2 will not have sufficient PP for adequate
breathing, so unpressurized aircraft cannot safely operate at anything close
to that level (unless the occupants are equipped with pressure suits).

This also explains why cabin depressurization above 30000 feet is an
emergency requiring (a) the immediate donning by the pilot(s) of a pressure
mask and (b) emergency descent to an altitude where the conventional
emergency masks used by passengers will be sufficient.

--
-Elliott Drucker