Four stages of hypoxia

With increasing altitude and decreasing O₂ partial pressure the body passes through four hypoxia stages, well documented in aviation medicine (ICAO Doc 8984, FAA AC 61-107B):

Stage 1 — Indifferent Stage (0–3 000 ft / 0–900 m)

O₂ saturation: 95–98%.

Symptoms: practically none. A healthy pilot notices no difference from sea level.

Only measurable effect: a slight decrease of night vision from about 5 000 ft (retinal cones are O₂-sensitive) — therefore night flights above 5 000 ft are recommended with oxygen.

Stage 2 — Compensatory Stage (3 000–10 000 ft / 900–3 000 m)

O₂ saturation: 87–95%.

Symptoms: still subjectively unobtrusive, but the body compensates through:

Higher breathing rate and depth (reflex hyperventilation),
Higher heart rate (tachycardia),
Higher cardiac output.

Impairments the pilot does not notice:

Reaction time slightly increased,
Night vision noticeably reduced,
Concentration slightly reduced.

Practice: on VFR flights up to 10 000 ft the pilot feels "normal", but performance is silently degraded. On long flights at this altitude fatigue can build up.

Stage 3 — Disturbance Stage (10 000–15 000 ft / 3 000–4 500 m)

O₂ saturation: 80–87%.

Symptoms become noticeable:

Subjective:

Light-headedness,
Headache,
Fatigue, sleepiness,
Euphoria or anxiety (paradox — pilot feels "good" despite being impaired),
Tingling in extremities,
Visual disturbance (blurred, tunnel vision),
Hot flushes or chills.

Objectively measurable:

Reaction time noticeably increased (50–100% longer),
Judgement impaired — pilot makes poor decisions,
Writing, speaking, arithmetic slower and more error-prone,
Hand-eye coordination worse,
Short-term memory reduced.

Insidious: the pilot often doesn't notice the impairment — the euphoric component can even make them feel "better" than normal.

Stage 4 — Critical Stage (above 15 000 ft / 4 500 m)

O₂ saturation: < 80%.

Symptoms:

Severe judgement impairment,
Spasmodic movements,
Unconsciousness within minutes,
In extreme cases: respiratory and cardiac arrest.

Time of Useful Consciousness (TUC) drops rapidly:

18 000 ft: 20–30 min.
22 000 ft: 5–10 min.
25 000 ft: 3–5 min.
30 000 ft: 1–2 min.
35 000 ft: 30–60 s.
40 000 ft: 15–20 s.

In a rapid decompression (pressurised-cabin event) TUC drops dramatically further (factor 2–3) due to reverse diffusion of O₂ from blood back to alveoli.

Practical consequences for PPL(A)

Thresholds

5 000 ft AGL at night: O₂ recommended for night vision (FAA recommendation).
10 000 ft cabin altitude: first symptoms — pilot should be alert.
12 500 ft (Part-NCO) / 13 000 ft (FAA): O₂ for crew required.
PPL types without pressurised cabin should normally operate below 10 000 ft.

Recognising one's own hypoxia

Important: the pilot learns personal symptoms through hypobaric chamber training (in an aero-medical institute). This is not mandatory for PPL(A) in the EU, but strongly recommended.

In flight: any unusual feeling at altitude — descend immediately and use O₂ if available.