I don't know how many divers there are among you. Me as a technical diver work under pressure everytime I dive
First - partial pressures. Lets assume that there is roughly 1at of pressure on the surface of earth. So the partial pressure of oxygen is 0,21at in air compared to pure oxygene. If we go down in the water, then partial pressure increases. at the depth of 40m oxygene partial pressure would be 1,05at -it is like breathing a pure oxygene.
If we go up, then at FL450 air pressure is 0.151at. partial pressure of oxygene would be 0.03171at or it would be equivalent of air with 3,17% oxygene content on sea level. Almost nothing. That would retard fire considerably. How much, I couldn't tell. Perhaps there are firemen who know that.
What I know is that oxygene pressures should not drop below 0.16at for human to remain normally functional. Breathing pure oxygen should leave pilots concious if they are fit enough. But 0.15at is at the edge, some may feel hypoxia signs.
Yet there is another problem. There is a nitrogen part in the air we breathe. if surrounding pressure drops too quickly, nitrogen dissolved in our tissues starts to move out. If that process is quick enough, it causes small bubbles in tissues and bloodstream. Bubbles block small bloodvessels. It is called Decompression sickness. If severe enough, it could cause unconciousness in worst case as brain becomes oxygene deprivated due to the blocked capillaries. DCS could paralyse, cause pain, numbness, vision distortion etc. So it is not desired while piloting AC.
That is why, I think rapid descent to much lower level is necessary than it would be necessary because of oxygen issues.
At FL250 air pressure is 0,384at. with oxygene partial pressure 0.08at it is like breathing air with 8% oxygene content at sea level. That is not enough to remain concious. Breathing oxygene is a must. It is enough to breathe oxygene without pressurisation.
Compared to pressure maintained in the cabin during flight - 0.797at, 0.384 is a drop in pressure almost 2x . In diving this kind of quick change in ambient pressures normally does not cause DCS.
Hope this help.