It is commonly held that a .12 partial pressure of oxygen (in atmospheres) is the threshold for serious hypoxia. (.10 atm is below the threshold for life support)
The stated .12 atm PP of O2 breathing air occurs at 15000msl, which -of course- coincides with the applicable FAR. (.10 atm PP of O2 would occur at 20,000 ft)
Breathing Pure O2 at around 48000msl would be the upper limit for breathing pure O2 at the ambient atmospheric pressure. The partial pressure of O2 would be 100% of the atmospheric pressure, which is .12 atm at that altitude
A 2.82 atm partial pressure of oxygen (pure O2 at a 60 fsw equivalent in a hyperbaric chamber) is the medical limit for administering O2 to DCS sufferers. This can only be tolerated by the body for around ten minutes. At these concentrations, O2 will begin to oxidize lung and nervous system tissues. When that happens, it is called Pulmonary or CNS Oxygen Toxicity.
The non-decompression limit for -recreational- divers breathing air is 132 feet of salt water and eight minutes at that depth. That's 8 minutes from beginning the descent until beginning the ascent. That would mean only 1 minute or so at a depth of 132fsw (which is 5 atmospheres btw), depending on how fast you sink. That limit is based on nondecompression dives, and the limiting factor is nitrogen absorption. If we switch to saturation diving, the navy exceptional exposure limit is 1.8 atm PP of O2. That would occur at 8.5 atm breathing air, or 125psi which equates to ~250 feet of salt water.
If there are any rec/nitrox divers who don't understand this, email me.