Hypoxia is a deficiency of oxygen reaching the body's tissues, and it is the single deadliest threat of high-altitude flight because it impairs the very judgment needed to recognize it.
Hypoxia is insidious because it produces a false sense of well-being (euphoria) while degrading performance. Common signs include cyanosis (blue fingernails/lips), headache, tingling, light-headedness, tunnel vision, slowed thinking, and impaired judgment. Night vision is the first thing to degrade — measurable deterioration begins as low as 5,000 ft. Symptoms are individual and consistent for each person, which is why altitude-chamber or reduced-oxygen training is so valuable: it lets a pilot learn their own signature symptoms.
The time of useful consciousness (TUC) — the interval between an oxygen-deprivation event and the loss of effective performance — shrinks dramatically with altitude: roughly 20–30 minutes at FL180, 5–12 minutes at FL250, 3–5 minutes at FL300, and only 30–60 seconds at FL400 (and as little as 15–20 seconds above FL430). A rapid decompression cuts TUC to roughly one-third to one-half of these values because gas is forced out of the lungs.
If hypoxia is suspected: don oxygen immediately, descend, and reduce workload. In a pressurization failure the memory-item sequence is to don the quick-donning mask first, establish crew communication, and initiate an emergency descent. Treat any unexplained cognitive or visual symptom at altitude as hypoxia until proven otherwise.