Nope, it's pressure altitude. In other words how much pressure is pushing oxygen down in your lungs

No. Should be pressure altitude.

Consider this: the air inside your lungs is always at ambient pressure (plus a very small extra for breathing effort), but it is at 37 degrees, whatever the ambient temperature.

If you inhale air which is +50 at your lips and nostrils, it cools down in your mouth, nose and throat, and contracts.

If you inhale air which is -25 at your lips and nostrils, it will warm up in the nose, mouth and throat. Although it will be uncomfortable. You can live and breathe at -25 or indeed -50 - you would do well to protect your ears, cheeks, nose etc. from frostbite - but when you try to pant, the cold air in your throat and lungs is irritating and causes coughing.

An airplane may be flying unpressurized at 3000 metres and +25 Celsius in Tibet, Afghanistan, Andes, tropical New Guinea... Or it may be flying at 3000 metres and -25 Celsius over Canada in winter.

The airframe will perform far better at -25 Celsius - the wings have more air to lift them, the propeller has more air to bite into, the engine has more air to burn.

But the humans inside - at +25 Celsius, they are free to pant. At -25 Celsius, their breathing is restricted by the discomfort from cold air.

So... how do people endure the challenges from combination of low air pressure and low temperature?

Not having had the opportunity to go above 8,500feet in an unpressurised a/c, let alone around the 10,000 feet mark I can't comment directly on those effects, other than what is taught in HPL. However having spent a few years in the north working on tourist boats, including a diving boat, I did notice that air consumption and getting "narced" (Nitorgen Narcosis) varied among individuals, with people with large muscle mass often consuming oxygen more quickly, so it is really up to the individual.

Curious as to whether someone with a short and stocky build also can tollerate higher altitudes, as well as G load (vs tall skinny), or whether thats just because the heart has less distance to pump :p

would this be why a dash 8 doesnt have passenger oxy masks as standard? is the theory that they can only go to FL250 and useful brain function at that level is around the 1-2 minute mark so by the time the crew starts a emergency decent and the pax fumble getting their oxy masks on they will already be down around the 12-14,000ft mark?

No she didn't. I'm reliably informed though that I would be much better liked if she had. I once took her to task on the matter, at which point she congratulated me for being a smarty pants.

I note that a few other share the weakness though!

Gav - there is actually a variant that can go to FL280 or so, but it has passenger masks fitted. As I understand it, the loss in payload doesn't really justify the added fuel efficiency.