I was interested to see, when I landed, that I had two incorrect answers out of three. On the other hand, my measured oxygen saturation — the percent of the oxygen-holding capacity of the blood that is actually being used — had been higher at all altitudes than the standard chart said it should be. (Of course, the oximeter could be wrong.) I was interested too to see that the saturation lagged by a minute or so as I climbed or descended; the often-heard statement that “recovery is immediate” with administration of supplemental oxygen may apply to waist gunners whose masks came unplugged, but it does not strictly apply to saturation percentages.
Fingertip oximetry does not tell how much oxygen is actually getting to the brain, but it’s better than nothing. The real problem is correlating saturation with impairment. Furgang recommends going on oxygen when saturation, normally 98 to 99 percent at ground level, drops below 92 to 93 percent in daytime or 94 to 95 percent at night. A study of workers building an observatory at the 16,000-foot level in Chile identifies “minimal mental impairment” with a range of oxygen saturations between 85 percent and 75 percent, which correspond, on average, to altitudes between 10,000 feet and 13,000 feet (but your results may vary). One study (McFarland, 1972), with specious exactitude, gives a table of percentages of sea level performance in various categories. For example, at 13,650 feet (4,200 meters) visual sensitivity — this would correspond to night vision — retains only 56 percent of its sea level value; short-term memory drops to 83 percent and arithmetic ability to 92 percent. Interestingly, the last thing to go was found to be “decision-making ability,” which was at 95 percent at 13,650 and 90 percent at 16,250 feet (5,000 meters). This should be a comfort to scofflaws, since decision-making ability — assuming that this means making the right decision, not just any decision — would seem to be the factor with the greatest relevance to flight safety.
One well-known aspect of hypoxia is that a hypoxic person feels euphoric and is unaware of any impairment. It is therefore pointless to say to yourself, “I will descend (or put on the mask or cannula) as soon as I start to feel odd.” You won’t feel odd. You have to accept as a given that once you are at 10,000 feet or higher, you are already hypoxic. That doesn’t mean that you are incapable of flying an airplane, but you may be incapable of reliable self-assessment. An oximeter is valuable because it provides a measure of objectivity — and it doesn’t hurt to have some oxygen aboard, just in case.
For more on hypoxia, check out "Aftermath: Breathless" and "Flying Higher than Usual? Consider the Hypoxia Threat."