Originally Posted by
BackPacker
As the whole problem of hypoxia is air pressure related, and FLs are defined using a constant pressure, it makes a lot more sense to define the oxygen requirements as FL than as altitudes.
Unfortunately in the US the transition altitude is well above the altitude (or FL, whatever you prefer) so it makes sense for the US to express the oxygen requirements as altitudes.
It is however the human body which has the casting vote.
It defines the oxygen requirements in terms of density altitude.
P