Time of useful consciousness (TUC), also effective performance time (EPT), is defined as the amount of time an individual is able to function effectively (e.g. perform
flying duties) in an environment of inadequate
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
supply.
It is the period of time from the interruption of the oxygen supply or exposure to an oxygen-poor environment to the time when useful function is lost, and the individual is no longer capable of taking proper corrective and protective action. It is not the time to total
unconsciousness
Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an consciousness, awareness of self and environment or to respond to any human or environmental Stimulus (physiology), stimulus. ...
. At the higher altitudes, the TUC becomes very short; considering this danger, the emphasis is on prevention rather than cure.
For orbital altitudes and above, that is, direct exposure to space, 6–9 seconds of consciousness is expected.
Medical analysis and variations
There are many individual variations of
hypoxia, even within the same person. Generally,
old age
Old age refers to ages nearing or surpassing the life expectancy of human beings, and is thus the end of the human life cycle. Terms and euphemisms for people at this age include old people, the elderly (worldwide usage), OAPs (British usage ...
tends to reduce the efficiency of the pulmonary system, and can cause the onset of hypoxia symptoms sooner.
Smoking
Smoking is a practice in which a substance is burned and the resulting smoke is typically breathed in to be tasted and absorbed into the bloodstream. Most commonly, the substance used is the dried leaves of the tobacco plant, which have bee ...
drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by . Hypoxia can be produced in a
hypobaric chamber
A hypobaric chamber, or altitude chamber, is a chamber used during aerospace or high terrestrial altitude research or training to simulate the effects of high altitude on the human body, especially hypoxia (low oxygen) and hypobaria (low ambient ...
. This can be useful for identifying individual symptoms of hypoxia, along with rough estimates of the altitude that causes problems for each person. Identifying symptoms is often helpful for self-diagnosis in order to realize when altitude should be reduced.
The table below shows average TUCs as documented by the
Federal Aviation Administration
The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
; a rapid ascent results in a lower TUC. The TUCs for any given individual may differ significantly from this.
Aerobic exercise
Aerobic exercise (also known as endurance activities, cardio or cardio-respiratory exercise) is physical exercise of low to high intensity that depends primarily on the aerobic energy-generating process. "Aerobic" is defined as "relating to, inv ...
during the TUC period will reduce the TUCs considerably; so will exercise immediately prior to the TUC as this induces an
oxygen debt prior to exposure.
See also
*
Armstrong limit
The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of ...
maximum survivable altitude
*
Cabin pressurization
Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is ...
*
Pressure suit
A pressure suit is a protective suit worn by high-altitude pilots who may fly at altitudes where the air pressure is too low for an unprotected person to survive, even breathing pure oxygen at positive pressure. Such suits may be either full-pr ...
*
Uncontrolled decompression
Uncontrolled decompression is an unplanned drop in the pressure of a sealed system, such as an aircraft cabin or hyperbaric chamber, and typically results from human reliability, human error, fatigue (material), material fatigue, engineering failur ...
References
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Aviation medicine
Pulmonology
Aviation accidents and incidents