Maybe I'll rename the thread so we're not off topic.
First blush, the only thing I would add to Cary's comments is that there are important individual differences which, and rightly so, would be too complex for the FAA to embed in policy.
One difference worth mentioning is the MSL altitude to which your physiology has accommodated.
I have lived at higher altitudes for most of my life. For the last five years, I have lived at 5,500 MSL and exercise up to 8.500 MSL. My son (also a pilot) has lived most of his adult life near sea level. Because it is often necessary for me to fly at 11,500 MSL to provide the 2,000 ft. AGL clearance I prefer in high mountain winds, I always carry an oximeter and generally have O2 on board. My son and I have seen interesting differences when we have flown together. While his O2 saturation readings begin dropping at 7,500 MSL, mine don't show much change until 12,500 MSL and even then they are in the mid-90's. My son also reports common symptoms like headache, irritability, yawning, etc.
So, I add O2 when required by the FAA regs but I have never experienced any symptoms from flying for four hours without O2 at 11,500. There is more to this but my main point is that a pilot's acclimated MSL altitude is a factor.
It would be interesting to hear from others who have lived at least a few years at high altitude (it takes about that long for all factors to adapt) about their altitude tolerance.
On a related note: pulse oximeters -- even the good ones and not all are good -- do not offer information about hemoglobin concentration, cardiac output, efficiency of oxygen delivery to the tissues, oxygen consumption, sufficiency of oxygenation, and other factors that are the real drivers of some of the underlying safety issues. They do give you a decent estimate of
changes in your oxygen saturation that are almost certainly a decent estimate of changes in the factor that is actually important, pulmonary arterial saturation which, in turn, determines how well you brain and nervous system will function in relation to optimum parameters. My recommendation is to buy a oximeter that comes with a lab calibration sheet. These days, most of them are probably OK but you want one that has been individually tested.
P.S. My dog has always lived at 5,500 MSL and he arrives even fresher than I do, probably because he sleeps until he feels the wheels on touchdown.
I've been checking on winds daily to get an average and my software says there is a good chance that my best altitude will be 14,500 - 15,500 with O2.
