An altimeter is a barometer. It measures pressure plus how much you turned the sub-scale knob.
That's exactly the point I was trying to make.
If you define the hypoxia air pressure (assuming there is one) as a flight level, you can simply set the subscale to 1013, read the flight level and see if you're in the danger zone or not. Whereas if you define the hypoxia danger zone as an altitude, you need to correct for the subscale setting, but backwards from what we're used to. So it makes a lot more sense to define the hypoxia/oxygen limits as flight levels instead of altitudes.
Going along with Carrots example. Suppose for a moment that 700 hPa is the start of the danger zone. If the air pressure is 700.0 hPa or lower you run an hypoxia risk, if the air pressure is 700.0 hPa or higher you're fine. (I know it's not such a black and white issue, but let's go along with that for now.)
We have no way of knowing the actual air pressure in the aircraft unless we bring an actual barometer. But, hey, our altimeter is a barometer too, just with a scale that works the other way round (increasing air pressure leads to a reduction of the readout) and its datum is adjustable.
Flight Levels are defined against a "QNH" of 1013. In other words, FL000 = 1013 hPa, FL001 = 1010 hPa, FL002 = 1007 hPa and so forth. And since that relation is not entirely linear and since I don't have the ISA atmosphere definitions to hand, let's assume FL100 = 700 hPa. This means I can use my altimeter to find out whether I'm in the danger zone or not: Set the subscale to 1013, see if I'm above 10.000' on the dial and I'm in the danger zone.
But as soon as you start setting a different subscale, because you insist on flying altitudes rather than flight levels, 700 hPa air pressure no longer corresponds to a reading of 10.000' on the altimeter. It corresponds to something that may be a few 100s of feet lower or higher. So you need to correct for that correction again.
As an example, suppose the QNH is 1007 and you have that set on the dial. This means that zero feet on the dial now corresponds to a barometric pressure of 1007 hPa. 100 feet corresponds to 1004 hPa, 200 feet corresponds to 1001 hPa and so forth. And the 700 hPa level no longer corresponds to the readout of 10.000'. Instead, the 700 hPa hypoxia danger level is now found where the altimeter reads 9.800 feet.
So on a low pressure day you may be in danger of hypoxia even below 10.000 feet altitude. Conversely, a high pressure day will give you a few hundred feet above 10.000 feet where you will not suffer from hypoxia. (Again, assuming it's black and white, which it isn't.)
Now try calculating that when you're already suffering from hypoxia.
