Editing Altitude sickness (section)

==Cause==
{{See also|Breathing#Breathing at altitude|Effects of high altitude on humans}}
[[File:Mount Everest as seen from Drukair2 PLW edit.jpg|thumb|Climbers on [[Mount Everest]] often experience altitude sickness.]]
Altitude sickness can first occur at {{convert|1500|m|ft}}, with the effects becoming severe at extreme altitudes (greater than {{convert|5500|m|ft}}). Only brief trips above {{convert|6000|m|ft}} are possible and supplemental oxygen is needed to avert sickness.

As altitude increases, the available amount of oxygen to sustain mental and physical alertness decreases with the overall air pressure, though the relative percentage of oxygen in air, at about 21%, remains practically unchanged up to {{convert|21000|m|ft}}.<ref>{{Cite journal |author=FSF Editorial Staff |url=http://flightsafety.org/hf/hf_may-jun97.pdf |title=Wheel-well Stowaways Risk Lethal Levels of Hypoxia and Hypothermia |journal=[[Human Factors and Aviation Medicine]] |page=2 |volume=44 |issue=3 |date=May–June 1997 |publisher=[[Flight Safety Foundation]] |access-date=28 October 2010 |quote=The relative amount of oxygen in the air (21 percent) does not vary appreciably at altitudes up to 21,350 meters (70,000 feet). |archive-url=https://web.archive.org/web/20101128161251/http://flightsafety.org/hf/hf_may-jun97.pdf |archive-date=28 November 2010 |url-status=live}}</ref> The [[Root mean square speed|RMS velocities]] of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen until stratospheric heights.

[[Dehydration]] due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.<ref>{{cite journal |vauthors=Hackett PH, Roach RC |title=High-altitude illness |journal=[[The New England Journal of Medicine]] |volume=345 |issue=2 |pages=107–14 |date=July 2001 |pmid=11450659 |doi=10.1056/NEJM200107123450206}}</ref>

The rate of ascent, altitude attained, amount of physical activity at high altitude, as well as individual susceptibility, are contributing factors to the onset and severity of high-altitude illness.

Altitude sickness usually occurs following a rapid ascent and can usually be prevented by ascending slowly.<ref name=Thompson/> In most of these cases, the symptoms are temporary and usually abate as altitude acclimatization occurs. However, in extreme cases, altitude sickness can be fatal.

High altitude illness can be classified according to the altitude: high ({{convert|1500|–|3500|m|ft}}), very high ({{convert|3500|–|5500|m|ft}}) and extreme (above {{convert|5500|m|ft}}).<ref name=":1">{{cite journal |vauthors=Molano Franco D, Nieto Estrada VH, Gonzalez Garay AG, Martí-Carvajal AJ, Arevalo-Rodriguez I |title=Interventions for preventing high altitude illness: Part 3. Miscellaneous and non-pharmacological interventions |journal=The Cochrane Database of Systematic Reviews |volume=4 |issue=4 |pages=CD013315 |date=April 2019 |pmid=31012483 |pmc=6477878 |doi=10.1002/14651858.CD013315}}</ref>

=== High altitude ===
At high altitude, {{convert|1500|to|3500|m|ft}}, the onset of physiological effects of diminished inspiratory oxygen pressure (PiO<sub>2</sub>) includes decreased exercise performance and increased ventilation (lower arterial [[partial pressure of carbon dioxide]]: PCO<sub>2</sub>). While arterial oxygen transport may be only slightly impaired the [[Oxygen saturation (medicine)|arterial oxygen saturation]] (SaO<sub>2</sub>) generally stays above 90%. Altitude sickness is common between {{convert|2400|and|4000|m|ft}} because of the large number of people who ascend rapidly to these altitudes.<ref name="Auerbach 2007">{{cite book |vauthors=Auerbach P |author-link=Paul Auerbach |title=Wilderness Medicine |publisher=Mosby Elsevier |edition=5th |year=2007 |isbn=978-0-323-03228-5 |ref=Auer07}}</ref>

=== Very high altitude ===
At very high altitude, {{convert|3500|to|5500|m|ft}}, maximum SaO<sub>2</sub> falls below 90% as the arterial PO<sub>2</sub> falls below 60mmHg. Extreme [[hypoxemia]] may occur during exercise, during sleep, and in the presence of high altitude pulmonary edema or other acute lung conditions. Severe altitude illness occurs most commonly in this range.<ref name="Auerbach 2007"/>

=== Extreme altitude ===
Above {{convert|5500|m|ft}}, marked hypoxemia, [[hypocapnia]], and [[alkalosis]] are characteristic of extreme altitudes. Progressive deterioration of physiologic function eventually outstrips acclimatization. As a result, [[List of highest settlements|no permanent human habitation occurs above {{convert|6000|m}}]]. A period of acclimatization is necessary when ascending to extreme altitude; abrupt ascent without supplemental oxygen for other than brief exposures invites severe altitude sickness.<ref name="Auerbach 2007"/>