Can you on forehand tell which of the pilots that are more likely to be affected?
Can't think of any test to predict susceptibilty.
At a guess, I'd say the usual suspects of age, frailty and health problems would play a part- it's a long time since I opened a physiology text book, but you could look up the "oxygen saturation curve," if you want to find out more.
Or ask slim slag, he's a bit nurdish- he'll probably know.
The bottom line though, if in doubt check it out, it's not worh theorising with. Do you have a different agenda martin?