Time of useful consciousness
{{Short description|Duration of effective performance in a hypoxic environment}}
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.{{cite book |title=Fundamentals Of Aerospace Medicine: Translating Research Into Clinical Applications, 3rd Rev Ed. |last=Dehart |first=R. L. |author2=J. R. Davis |year=2002 |publisher=Lippincott Williams And Wilkins |location=United States |isbn=978-0-7817-2898-0 |page=720 }} 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, 6–8 seconds of consciousness is expected.Geoffrey A. Landis, [http://www.geoffreylandis.com/vacuum.html Human Exposure to Vacuum] web page
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.{{cite journal |vauthors=Yoneda I, Tomoda M, Tokumaru O, Sato T, Watanabe Y |title=Time of useful consciousness determination in aircrew members with reference to prior altitude chamber experience and age |journal=Aviat Space Environ Med |volume=71 |issue=1 |pages=72–6 |date=January 2000 |pmid=10632134 }} Smoking drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by {{convert|1,000-2,000|m|ft}}.{{cite journal |vauthors=Yoneda I, Watanabe Y |title=Comparisons of altitude tolerance and hypoxia symptoms between nonsmokers and habitual smokers |journal=Aviat Space Environ Med |volume=68 |issue=9 |pages=807–11 |date=September 1997 |pmid=9293349 }} 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.{{Cite web | url=https://www.faa.gov/pilots/training/airman_education/media/AC%2061-107A.pdf | title=AC 61-107A - Operations of aircraft at altitudes above 25,000 feet MSL and/or mach numbers (MMO) greater than .75 | archive-url=https://web.archive.org/web/20150213092047/https://www.faa.gov/pilots/training/airman_education/media/AC%2061-107A.pdf | archive-date=2015-02-13}} 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.{{cite web|url=http://www.theairlinepilots.com/medical/decompressionandhypoxia.htm|title=Cabin Decompression and Hypoxia|author=Mark Wolff|date=2006-01-06|access-date=2008-09-01|publisher=theairlinepilots.com}}
| class="wikitable" |
| Altitude (measured barometrically)
!TUC (normal ascent) !TUC (rapid decompression) |
|---|
| {{Flight level|180|prefix=lk}}
|20 to 30 minutes |10 to 15 minutes |
| {{Flight level|220}}
|10 minutes |5 minutes |
| {{Flight level|250}}
|3 to 5 minutes |1.5 to 3.5 minutes |
| {{Flight level|280}}
|2.5 to 3 minutes |1.25 to 1.5 minutes |
| {{Flight level|300}}
|1 to 2 minutes |30 to 60 seconds |
| {{Flight level|350}}
|30 to 60 seconds |15 to 30 seconds |
| {{Flight level|400}}
|15 to 20 seconds |7 to 10 seconds |
| {{Flight level|430}}
|9 to 12 seconds |5 to 6 seconds |
| {{Flight level|500}}
|8 to 10 seconds |5 seconds |
See also
- Armstrong limit maximum survivable altitude
- Cabin pressurization
- Pressure suit
- Uncontrolled decompression