PaulH wrote:
I took my Arrow up to 10,500 today to check out my fingertip O2
measuring device. I can maintain 90-93% saturation with deep
breathing and no supplemental O2. Anybody know what the minimum safe
level is for daytime? I guess it might be cumulative, i.e. the longer
you go at 92% the less safe it is? Dropped down to about 88% when I
got distracted with some cockpit chores and started normal sea level
breathing.

Once your oxygen saturation gets to about 90% you are at a point where
any further decrease in ambient oxygen pressure can cause a big drop in
the saturation. So, for instance, a decrease in atmospheric pressure
might significantly decrease the amount of oxygen in your blood. For
this reason, people usually talk about a saturation of 90% as the
minimum safe level.

However, for a healthy person at rest, an oxygen saturation in the mid
80s is likely to be adequate for most people to feel okay and think
normally. (That is not to say that it's a good idea while flying to
intentionally let it get that low rather than use supplemental oxygen.)

In contrast, I'm not so comfortable with "deep breathing" to maintain a
higher oxygen saturation. In accomplishing this, you are almost
certainly decreasing the carbon dioxide in your blood, and that can make
you dizzy, lightheaded, anxious, and/or confused.

--
David Rind