Upto 10,000 ft air only
10,000 - 33,700 ft Oxygen/air mix
33,700 - 40,000 ft 100% oxygen
40,000 + 100% Oxygen under pressure



I flew fighters for the Air Force (F-4E) and I recal going to pressure
breathing at about 25,000 feet cabin altitude. This was an emergency
only
because the cabin pressure never got this high. I also recall a rule to
not go over 50,000 feet because of blood boiling (bends) IF the cabin
pressure was lost at greater than 50,000 feet.

Well the bends more commonly known as Decompression Sickness (DCS) is a
possibility from about 18,000 ft unpressurised and is caused by the
nitrogen
in the blood coming out.

-----remainder snipped for brevity----------

I recall reading that the Blackbird crews (SR71 and U2) breathed pure oxygen
for several hours prior to each mission in order to clear their bodies of
dissolved nitrogen. I presume that the above was the reason.

Peter