Editing Cabin pressurization (section)

===Spacecraft===
Russian engineers used an air-like nitrogen/oxygen mixture, kept at a cabin altitude near zero at all times, in their 1961 [[Vostok (spacecraft)|Vostok]], 1964 [[Voskhod (spacecraft)|Voskhod]], and 1967 to present [[Soyuz (spacecraft)|Soyuz]] spacecraft.<ref>{{cite book
 | last = Gatland
 | first = Kenneth
 | title = Manned Spacecraft
 | publisher = MacMillan
 | edition = Second
 | date = 1976
 | location = New York
 | page = 256
 }}
</ref> This requires a heavier [[space vehicle]] design, because the spacecraft cabin structure must withstand the stress of 14.7 pounds per square inch (1 atm, 1.01 bar) against the vacuum of space, and also because an inert nitrogen mass must be carried. Care must also be taken to avoid [[decompression sickness]] when cosmonauts perform [[extravehicular activity]], as current soft [[space suit]]s are pressurized with pure oxygen at relatively low pressure in order to provide reasonable flexibility.<ref>Gatland, p. 134</ref>

By contrast, the United States used a pure oxygen atmosphere for its 1961 [[Project Mercury|Mercury]], 1965 [[Project Gemini|Gemini]], and 1967 [[Apollo spacecraft]], mainly in order to avoid decompression sickness.<ref>{{cite book|last=Catchpole|first=John|title=Project Mercury – NASA's First Manned Space Programme|year=2001|publisher=Springer Praxis|location=Chichester, UK|isbn=1-85233-406-1|page=[https://archive.org/details/projectmercuryna0000catc/page/410 410]|url=https://archive.org/details/projectmercuryna0000catc/page/410}}</ref><ref>{{cite journal |last=Giblin |first=Kelly A. |date=Spring 1998 |title =Fire in the Cockpit! |journal=[[American Heritage of Invention & Technology]] |volume=13 |issue=4 |url=http://www.americanheritage.com/articles/magazine/it/1998/4/1998_4_46.shtml |archive-url=https://web.archive.org/web/20081120153024/http://www.americanheritage.com/articles/magazine/it/1998/4/1998_4_46.shtml |archive-date=November 20, 2008 |access-date=March 23, 2011}}</ref> Mercury used a cabin altitude of {{cvt|24800|ft}} ({{cvt|5.5|psi|bar}});<ref>Gatland, p. 264</ref> Gemini used an altitude of {{cvt|25700|ft}} ({{cvt|5.3|psi|bar}});<ref>Gatland, p. 269</ref> and Apollo used {{cvt|27000|ft}} ({{cvt|5.0|psi|bar}})<ref>Gatland, p. 278, 284</ref> in space. This allowed for a lighter space vehicle design. This is possible because at 100% oxygen, enough oxygen gets to the bloodstream to allow astronauts to operate normally. Before launch, the pressure was kept at slightly higher than sea level at a constant {{cvt|5.3|psi|bar}} above ambient for Gemini, and {{cvt|2|psi|bar}} above sea level at launch for Apollo), and transitioned to the space cabin altitude during ascent. However, the high pressure pure oxygen atmosphere before launch proved to be a factor in a fatal fire hazard in Apollo, contributing to the deaths of the entire crew of [[Apollo 1]] during a 1967 ground test. After this, [[NASA]] revised its procedure to use a nitrogen/oxygen mix at zero cabin altitude at launch, but kept the low-pressure pure oxygen atmosphere at {{cvt|5|psi|bar}} in space.<ref>{{Cite web | url=http://www.spacesafetymagazine.com/space-disasters/apollo-1-fire/ |title = The Apollo 1 Fire –}}</ref>

After the [[Apollo program]], the United States used "a 74-percent oxygen and 26-percent nitrogen breathing mixture" at {{cvt|5|psi|bar}} for [[Skylab]],<ref>{{cite book |editor1-last=Belew |editor1-first=Leland F. |title=SP-400 Skylab: Our First Space Station |date=1977 |publisher=NASA |location=Washington DC |page=18 |url=https://history.nasa.gov/SP-400/ch2.htm |access-date=July 15, 2019 |chapter=2. Our First Space Station}}</ref> and a cabin atmosphere of {{cvt|14.5|psi|bar}} for the [[Space Shuttle orbiter]] and the [[International Space Station]].<ref>{{cite book 
|first1=Michael L. 
|last1=Gernhardt 
|first2=Joseph P. 
|last2=Dervay 
|first3=James M. 
|last3=Waligora 
|first4=Daniel T. 
|last4=Fitzpatrick 
|first5=Johnny 
|last5=Conkin 
|title=EVA Operations 
|date=2013 
|publisher=NASA 
|location=Washington DC 
|page=1 
|chapter=5.4 Extravehicular Activities 
| chapter-url=https://www.nasa.gov/wp-content/uploads/2023/03/gernhardt-eva-ops-chp-5.4-2013.pdf
}}</ref>