Time of useful consciousness

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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 supply. [1] 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. 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. [2]

Medical analysis and variations

There are many individual variations of hypoxia, even within the same person. Generally, old age tends to reduce the efficiency of the pulmonary system, and can cause the onset of hypoxia symptoms sooner. [3] Smoking drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by 1,000–2,000 metres (3,300–6,600 ft). [4] Hypoxia can be produced in a hypobaric chamber. 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; a rapid ascent results in a lower TUC. [5] The TUCs for any given individual may differ significantly from this. Aerobic exercise 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. [6]

Altitude (measured barometrically)TUC (normal ascent)TUC (rapid decompression)
FL 180 (18,000 ft; 5,500 m) 20 to 30 minutes10 to 15 minutes
FL220 (22,000 ft; 6,700 m) 10 minutes5 minutes
FL250 (25,000 ft; 7,600 m) 3 to 5 minutes1.5 to 3.5 minutes
FL280 (28,000 ft; 8,550 m) 2.5 to 3 minutes1.25 to 1.5 minutes
FL300 (30,000 ft; 9,150 m) 1 to 2 minutes30 to 60 seconds
FL350 (35,000 ft; 10,650 m) 30 secs to 1 minute15 to 30 seconds
FL400 (40,000 ft; 12,200 m) 15 to 20 seconds7 to 10 seconds
FL430 (43,000 ft; 13,100 m) 9 to 12 seconds5 seconds
FL500 (50,000 ft; 15,250 m) 9 to 12 seconds5 seconds

See also

Related Research Articles

Hypoxia (medical) Medical condition caused by lack of oxygen in the tissues

Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during hypoventilation training or strenuous physical exercise.

Decompression sickness Disorder caused by dissolved gases emerging from solution

Decompression sickness is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompression. DCS most commonly occurs during or soon after a decompression ascent from underwater diving, but can also result from other causes of depressurisation, such as emerging from a caisson, decompression from saturation, flying in an unpressurised aircraft at high altitude, and extravehicular activity from spacecraft. DCS and arterial gas embolism are collectively referred to as decompression illness.

Breathing gas Gas used for human respiration

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural, breathing gas. But other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, high-altitude mountaineering, high-flying aircraft, submarines, space suits, spacecraft, medical life support and first aid equipment, and anaesthetic machines.

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 error, material fatigue, engineering failure, or impact, causing a pressure vessel to vent into its lower-pressure surroundings or fail to pressurize at all.

Altitude training

Altitude training is the practice by some endurance athletes of training for several weeks at high altitude, preferably over 2,400 metres (8,000 ft) above sea level, though more commonly at intermediate altitudes due to the shortage of suitable high-altitude locations. At intermediate altitudes, the air still contains approximately 20.9% oxygen, but the barometric pressure and thus the partial pressure of oxygen is reduced.

Oxygen mask Interface between the oxygen delivery system and the human user

An oxygen mask provides a method to transfer breathing oxygen gas from a storage tank to the lungs. Oxygen masks may cover only the nose and mouth or the entire face. They may be made of plastic, silicone, or rubber. In certain circumstances, oxygen may be delivered via a nasal cannula instead of a mask.

Cabin pressurization Process to maintain internal air pressure in aircraft

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 usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary before it is distributed to the cabin by one or more environmental control systems. The cabin pressure is regulated by the outflow valve.

Aerosinusitis Barotrauma of the sinuses

Aerosinusitis, also called barosinusitis, sinus squeeze or sinus barotrauma is a painful inflammation and sometimes bleeding of the membrane of the paranasal sinus cavities, normally the frontal sinus. It is caused by a difference in air pressures inside and outside the cavities.

Hyperoxia occurs when cells, tissues and organs are exposed to an excess supply of oxygen (O2) or higher than normal partial pressure of oxygen.

Diving disorders, or diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides into conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.

Hypoxemia Abnormally low level of oxygen in the blood

Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the body.

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 and hypobaria. Some chambers also control for temperature and relative humidity.

Armstrong limit Altitude above which water boils at human body temperature

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 consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. On Earth, the limit is around 18–19 km above sea level, above which atmospheric air pressure drops below 0.0618 atm. The U.S. Standard Atmospheric model sets the Armstrong pressure at an altitude of 63,000 feet (19,202 m).

Altitude decompression or hypobaric decompression is the reduction in ambient pressure below the normal range of sea level atmospheric pressure. Altitude decompression is the natural consequence of unprotected elevation to altitude, while hypobaric decompression is due to intentional or unintentional release of pressurisation of a pressure suit or pressurised compartment, vehicle or habitat, and may be controlled or uncontrolled, or the reduction of pressure in a hypobaric chamber.

Decompression practice Techniques and procedures for safe decompression of divers

The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in the tissues as a result of breathing at ambient pressures greater than surface atmospheric pressure, the equipment available and appropriate to the circumstances of the dive, and the procedures authorized for the equipment and profile to be used. There is a large range of options in all of these aspects.

Decompression theory Theoretical modelling of decompression physiology

Decompression theory is the study and modelling of the transfer of the inert gas component of breathing gases from the gas in the lungs to the tissues and back during exposure to variations in ambient pressure. In the case of underwater diving and compressed air work, this mostly involves ambient pressures greater than the local surface pressure, but astronauts, high altitude mountaineers, and travellers in aircraft which are not pressurised to sea level pressure, are generally exposed to ambient pressures less than standard sea level atmospheric pressure. In all cases, the symptoms caused by decompression occur during or within a relatively short period of hours, or occasionally days, after a significant pressure reduction.

Richard Deimel Vann is an American academic and diver.

Physiology of decompression The physiological basis for decompression theory and practice

The physiology of decompression involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues. Gas is breathed at ambient pressure, and some of this gas dissolves into the blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the tissues is in a state of equilibrium with the gas in the lungs,, or the ambient pressure is reduced until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

Human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.

The science of underwater diving includes those concepts which are useful for understanding the underwater environment in which diving takes place, and its influence on the diver. It includes aspects of physics, physiology and oceanography. The practice of scientific work while diving is known as Scientific diving. These topics are covered to a greater or lesser extent in diver training programs, on the principle that understanding the concepts may allow the diver to avoid problems and deal with them more effectively when they cannot be avoided.


  1. Dehart, R. L.; J. R. Davis (2002). Fundamentals Of Aerospace Medicine: Translating Research Into Clinical Applications, 3rd Rev Ed. United States: Lippincott Williams And Wilkins. p. 720. ISBN   978-0-7817-2898-0.
  2. Geoffrey A. Landis, Human Exposure to Vacuum web page
  3. Yoneda I, Tomoda M, Tokumaru O, Sato T, Watanabe Y (January 2000). "Time of useful consciousness determination in aircrew members with reference to prior altitude chamber experience and age". Aviat Space Environ Med. 71 (1): 72–6. PMID   10632134.
  4. Yoneda I, Watanabe Y (September 1997). "Comparisons of altitude tolerance and hypoxia symptoms between nonsmokers and habitual smokers". Aviat Space Environ Med. 68 (9): 807–11. PMID   9293349.
  5. https://www.faa.gov/pilots/training/airman_education/media/AC%2061-107A.pdf [ bare URL PDF ]
  6. Mark Wolff (2006-01-06). "Cabin Decompression and Hypoxia". theairlinepilots.com. Retrieved 2008-09-01.