Time of Useful Consciousness

Time of Useful Consciousness (TUC) — also called Effective Performance Time (EPT) — is the period following a sudden loss of oxygen supply at altitude during which a person retains the ability to take corrective or protective action. After TUC expires, cognitive and motor function deteriorate rapidly, leading to unconsciousness.

TUC varies dramatically with altitude:

• 22,000 ft: 5-10 minutes
• 25,000 ft: 3-5 minutes
• 30,000 ft: 1-2 minutes
• 35,000 ft: 30-60 seconds
• 40,000 ft: 15-20 seconds
• 43,000 ft: 9-12 seconds

These values assume a sudden decompression at rest. Physical exertion dramatically reduces TUC — a pilot fighting to control an aircraft during a rapid decompression at FL400 may have less than 10 seconds of useful consciousness. This is why the first action in a decompression emergency is always to don the oxygen mask immediately, before attempting any other action. Flight deck quick-donning masks can be put on with one hand in approximately 5 seconds.