Editing Human spaceflight (section)

==== Artificial atmosphere ====
There are two basic choices for an artificial atmosphere: either an Earth-like mixture of oxygen and an inert gas such as nitrogen or helium, or pure oxygen, which can be used at lower than standard atmospheric pressure. A nitrogen–oxygen mixture is used in the International Space Station and Soyuz spacecraft, while low-pressure pure oxygen is commonly used in space suits for [[extravehicular activity]].

The use of a gas mixture carries the risk of [[decompression sickness]] (commonly known as "the bends") when transitioning to or from the pure oxygen space suit environment. There have been instances of injury and fatalities caused by suffocation in the presence of too much nitrogen and not enough oxygen.
* In 1960, [[McDonnell Aircraft]] test pilot G.B. North passed out and was seriously injured when testing a Mercury cabin–space suit atmosphere system in a vacuum chamber, due to nitrogen-rich air leaking from the cabin into his space suit feed.<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 |publisher=American Heritage Publishing |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=20 November 2008 |access-date=23 March 2011}}</ref> This incident led NASA to decide on a pure oxygen atmosphere for the Mercury, Gemini, and Apollo spacecraft.
* In 1981, three pad workers were killed by a nitrogen-rich atmosphere in the aft engine compartment of the {{OV|102}} at the [[Kennedy Space Center Launch Complex 39]].<ref>[https://web.archive.org/web/20010605212352/http://www-lib.ksc.nasa.gov/lib/chrono.html 1981 KSC Chronology Part 1 – pages 84, 85, 100; Part 2 – pages 181, 194, 195], NASA</ref>
* In 1995, two pad workers were similarly killed by a nitrogen leak in a confined area of the [[Ariane 5]] launch pad at [[Guiana Space Centre]].<ref>[http://www.esa.int/esaCP/Pr_17_1995_p_EN.html "Fatal accident at the Guiana Space Centre"], ''ESA Portal'', 5 May 1993</ref>

A pure oxygen atmosphere carries the risk of fire. The original design of the Apollo spacecraft used pure oxygen at greater than atmospheric pressure prior to launch. An electrical fire started in the cabin of [[Apollo 1]] during a ground test at [[Cape Canaveral Air Force Station Launch Complex 34|Cape Kennedy Air Force Station Launch Complex 34]] on 27 January 1967, and spread rapidly. The high pressure, increased by the fire, prevented removal of the [[plug door]] hatch cover in time to rescue the crew. All three astronauts—[[Gus Grissom]], [[Ed White (astronaut)|Ed White]], and [[Roger Chaffee]]—were killed.<ref name="SP4029">{{cite book |last=Orloff |first=Richard W. |title=Apollo by the Numbers: A Statistical Reference |url=https://history.nasa.gov/SP-4029/SP-4029.htm |access-date=12 July 2013 |series=NASA History Series |orig-year=First published 2000 |date=September 2004 |publisher=NASA |location=Washington, D.C. |isbn=978-0-16-050631-4 |lccn=00061677 |id=NASA SP-2000-4029 |chapter=Apollo 1 – The Fire: 27 January 1967 |chapter-url=https://history.nasa.gov/SP-4029/Apollo_01a_Summary.htm}}</ref> This led NASA to use a nitrogen–oxygen atmosphere before launch, and low-pressure pure oxygen only in space.