Ever since I have tried to determine the minimum blood oxygen percentage which will just keep an average human conscious. It seems to be about 65 to 70 %. Does anyone out there have a more accurate figure? Climbers of Everest should know.
Having done hyperbaric training with a O2 sensor on in the last 3 months for my requal, I can tell you that by 70% you are symptomatic, (for me, tingling fingers and loss of colour perception) 60% your judgement is affected and by 50% you are on the verge of losing the chance to detect an issue.
The worst feeling was going onto 100% at the end of the run, the oxygen flushes the co2 out of the blood and your blood 02 initially takes a huge dive. The sense is pretty nasty and paradoxically if hypoxia you may be tempted to remove oxygen fearing that the supply is contaminated.