O2 safe level?

I acquired a Nonin O2 monitor a while back and Saturday took it out for
a test ride. I'm 68, mild heart disease, annual special issue medical,
exercise regularly.

With regular deep breathing, I was able to retain 90% or more O2 level
as indicated by the monitor up to 13,500, and felt fine. Given that
one of the early symptoms of hypoxia is euphoria, however, I'd
appreciate if any of you who have tested this stuff would share what
you know about where hypoxia begins for most people?