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Free ReferenceHypoxia awareness for every altitude

Time of Useful Consciousness

How long you have to take corrective action at altitude without supplemental oxygen. Memorize these numbers — they could save your life in a depressurization event.

TUC by altitude

AltitudeTUCNotes
18,000 ft (FL180)20 – 30 minutesOnset gradual; judgment degrades before you notice
22,000 ft (FL220)5 – 10 minutesNoticeable impairment; supplemental O2 required above FL180
25,000 ft (FL250)3 – 5 minutesRapid onset; don mask immediately in depressurization
30,000 ft (FL300)1 – 2 minutesVery rapid; seconds count
35,000 ft (FL350)30 – 60 secondsTypical jet cruise altitude; passenger masks deploy automatically
40,000 ft (FL400)15 – 20 secondsBarely time to reach for the mask
43,000 ft+ (FL430+)9 – 12 secondsNear-instant incapacitation; quick-donning masks essential

14 CFR 91.211 — Supplemental oxygen requirements

These are the FAA regulatory minimums for supplemental oxygen use. They apply to cabin altitude, not flight altitude — pressurized aircraft maintain lower cabin altitudes.

ConditionRequirement
Cabin altitude 12,500 – 14,000 ftFlight crew must use supplemental oxygen after 30 minutes above 12,500 ft cabin altitude
Cabin altitude above 14,000 ftFlight crew must use supplemental oxygen at all times above 14,000 ft cabin altitude
Cabin altitude above 15,000 ftEach occupant must be provided supplemental oxygen

Practical tips

  • Practice using your oxygen equipment on the ground. In a depressurization at FL350, you have 30–60 seconds.
  • Many pilots voluntarily use supplemental oxygen above 10,000 ft at night, because night vision degrades significantly with even mild hypoxia.
  • Pulse oximeters are inexpensive and give you a real-time SpO2 reading. Normal is 95–100% at sea level; below 90% at altitude demands immediate oxygen.
  • If you suspect hypoxia in a passenger or yourself, descend immediately and use oxygen. Do not wait to confirm symptoms.

Understanding hypoxia

Hypoxia is the state of insufficient oxygen reaching body tissues. At altitude, the partial pressure of oxygen decreases even though the percentage of O2 in the atmosphere remains 21%. Your hemoglobin simply cannot bind enough oxygen molecules at reduced pressure.

The insidious danger of hypoxia is that it impairs judgment before you recognize symptoms. Euphoria, tunnel vision, tingling, and cyanosis (blue fingernails) are classic signs — but by the time you notice them, your TUC may already be exhausted.

The values below assume a resting, healthy adult at sea-level acclimation. Physical exertion, smoking, alcohol, fatigue, and illness all reduce TUC significantly. Rapid decompression (versus slow leak) can halve TUC at any given altitude.

Altitude awareness, automated

AeroCopilot monitors your altitude and warns you about oxygen requirements and hypoxia risks.

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