O2 systems fail. Probably, the best thing you could do, would be go thru a altitude chamber course. Everyones symptoms are different and it is insidious while its happening. knowing your symptoms and when it(hypoxia) is happening to you, can be a life saver! I went threw a course at ASU a few years back. Recently while flying a friends glider in Moriarty, I noticed my symptoms gradually appearing (tingling o, the back of the neck and headache). This lead me to check the EDS system and realize it was not fuctioning properly. I was able to reset it and get it working so didnt abort but had I not recognized the problem------

CH