Editing Death zone (section)

== Physiological background ==
The [[human body]] has optimal endurance below {{cvt|150|m|ft}} elevation.<ref>{{cite journal|title=Effects of Altitude on Performance of Elite Track-and-Field Athletes|first1=MJ|last1=Hamlin|first2=WG|last2=Hopkins|first3=SC|last3=Hollings|journal=International Journal of Sports Physiology and Performance|year=2015|volume=10|issue=7 |pages=881–887|doi=10.1123/ijspp.2014-0261|pmid=25710483 }}</ref> The [[Atmospheric chemistry|concentration of oxygen]] (O<sub>2</sub>) in air is 20.9% so the [[partial pressure]] of O<sub>2</sub> (PO<sub>2</sub>) at sea level is about {{cvt|21.2|kPa|inHg psi}}. In healthy
individuals, this saturates [[hemoglobin]], the oxygen-binding red pigment in [[red blood cells]].<ref>{{cite web |url=http://www.flightstat.nonin.com/documents/Hypoxia,%20Oxygen%20and%20Pulse%20Oximetry.pdf |title=Hypoxia, Oxygen, and Pulse Oximetry|work=FlightState Pulse Oximeter |access-date=2006-12-29}}</ref>

Atmospheric pressure decreases with altitude while the O<sub>2</sub> fraction remains constant to about {{cvt|85|km}}, so PO<sub>2</sub> decreases with altitude as well. It is about half of its sea level value at {{cvt|5500|m|ft|-2}}, the altitude of the [[Everest base camps|Mount Everest base camp]], and less than a third at {{cvt|8849|m|ft}}, the summit of Mount Everest.<ref>{{cite web|url=http://www.physicalgeography.net/fundamentals/7d.html|title=Introduction to the Atmosphere|work=PhysicalGeography.net|access-date=2006-12-29}}</ref> When PO<sub>2</sub> drops, the body responds with [[altitude acclimatization]].<ref name=Acclimatization>{{cite journal |author=Muza, SR |author2=Fulco, CS |author3=Cymerman, A |title=Altitude Acclimatization Guide. |journal=US Army Research Inst. Of Environmental Medicine Thermal and Mountain Medicine Division Technical Report |issue=[[USARIEM|USARIEM-TN-04-05]] |year=2004 |url=https://www.usariem.army.mil/assets/docs/partnering/altitudeacclimatizationguide.pdf |archive-url=https://web.archive.org/web/20161020162036/http://www.usariem.army.mil/assets/docs/partnering/altitudeacclimatizationguide.pdf |url-status=dead |archive-date=October 20, 2016 |access-date=2009-03-05 }}</ref> Additional red blood cells are manufactured; the heart beats faster; non-essential body functions are suppressed, food digestion efficiency declines (as the body suppresses the [[digestive system]] in favor of increasing its cardiopulmonary reserves);<ref>{{ cite journal | last =Westerterp | first =Klaas | title =Energy and Water Balance at High Altitude | journal =News in Physiological Sciences | volume =16 | issue =3 | pages =134–137 | pmid =11443234 | url =http://physiologyonline.physiology.org/cgi/content/full/16/3/134 | date = June 1, 2001 | doi =10.1152/physiologyonline.2001.16.3.134 | s2cid =26524828 | url-access =subscription }}</ref> and one [[respiratory system|breathes]] more deeply and more frequently. But acclimatization requires days or even weeks. Failure to acclimatize may result in [[altitude sickness]], including [[high-altitude pulmonary edema]] ([[HAPE]]) or [[cerebral edema]] ([[HACE]]).<ref name=MedicalProblems>{{cite web |author=Cymerman, A |author2=Rock, PB |title=Medical Problems in High Mountain Environments. A Handbook for Medical Officers |publisher=US Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report |id=USARIEM-TN94-2 |url=https://apps.dtic.mil/sti/citations/ADA278095 |access-date=2009-03-05}}</ref><ref>{{cite web|url=http://www.princeton.edu/~oa/safety/altitude.html|title=Outdoor Action Guide to High Altitude: Acclimatization and Illnesses|work=Outdoor Action First Aid & Safety Training|access-date=2006-12-29}}</ref>

Humans have survived for 2 years at {{cvt|5950|m|ft|-2}} [{{convert|475|mbar|inHg psi}} of atmospheric pressure], which appears to be near the limit of the permanently tolerable highest altitude.<ref>{{cite journal|last=West|first=JB|pmid=12631426|title=Highest permanent human habitation|journal=High Altitude Medical Biology|volume=3|number=4|pages=401–407|year=2002|doi=10.1089/15270290260512882}}</ref> At extreme altitudes, above {{cvt|7500|m|ft|-2}} [{{convert|383|mbar|inHg psi}} of atmospheric pressure], sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly.<ref name=MedicalProblems/><ref name="pmid3215854">{{cite journal |last1=Rose|first1=MS|last2=Houston|first2=CS|last3=Fulco|first3=CS|last4=Coates|first4=G|last5=Sutton|first5=JR|last6=Cymerman|first6=A |title=Operation Everest. II: Nutrition and body composition |journal=J. Appl. Physiol. |volume=65 |issue=6 |pages=2545–51 |year=1988 |pmid=3215854 |doi= 10.1152/jappl.1988.65.6.2545|url=http://jap.physiology.org/cgi/pmidlookup?view=long&pmid=3215854 |access-date=2009-03-05|url-access=subscription}}</ref><ref name="pmid1483750">{{cite journal |author=Kayser B |title=Nutrition and high altitude exposure |journal=Int J Sports Med |volume=13 |pages=S129–32 |year=1992 |issue=Suppl 1 |pmid=1483750 |doi=10.1055/s-2007-1024616|s2cid=5787317 }}</ref>

In the death zone and higher, no human body can acclimatize. The body uses up its store of oxygen faster than it can be replenished.  An extended stay in the zone without [[Bottled oxygen (climbing)|supplementary oxygen]] will result in deterioration of body functions, loss of consciousness, and ultimately, death.<ref name="PBS"/><ref name=Huey/><ref name=Grocott/> Scientists at the High Altitude Pathology Institute in Bolivia dispute the existence of a death zone, based on observation of extreme tolerance to [[Hypoxia (medical)|hypoxia]] in patients with [[chronic mountain sickness]] and normal fetuses in-utero, both of which present pO<sub>2</sub> levels similar to those at the summit of Mount Everest.<ref name=nodeath>{{cite journal|last1=Zubieta-Castillo|first1=G.|last2=Zubieta-Calleja|first2=G.R.|last3=Zubieta-Calleja|first3=L.|last4=Zubieta-Castillo|first4=Nancy|title=Facts that Prove that Adaptation to life at Extreme Altitude (8842m) is possible|journal=Adaptation Biology and Medicine|year=2008|volume=5|issue=Suppl 5|pages=348–355 |url=http://zuniv.net/pub/Everest2.pdf}}</ref>