Editing Space suit (section)

=== Operating pressure ===
[[File:Prebreathe.jpg|thumb|Astronaut [[Steve MacLean (astronaut)|Steven G. MacLean]] pre-breathes prior to an EVA]]
Generally, to supply enough oxygen for [[Respiratory system|respiration]], a space suit using pure oxygen must have a pressure of about {{convert|32.4|kPa|Torr psi|sigfig=2|abbr=on}}, equal to the {{convert|20.7|kPa|Torr psi|sigfig=2|abbr=on}} [[partial pressure]] of oxygen in the [[Earth's atmosphere]] at sea level, plus {{convert|5.3|kPa|Torr psi|sigfig=2|abbr=on}} {{CO2}}{{citation needed|date=May 2020}} and {{convert|6.3|kPa|Torr psi|sigfig=2|abbr=on|lk=on}} [[water vapor]] pressure, both of which must be subtracted from the [[Pulmonary gas pressures|alveolar pressure]] to get alveolar oxygen partial pressure in 100% oxygen atmospheres, by the [[alveolar gas equation]].<ref>{{cite web |url=http://www.globalrph.com/martin_4_most2.htm |title=The Four Most Important Equations In Clinical Practice |last=Martin |first=Lawrence |website=GlobalRPh |publisher=David McAuley |access-date=June 19, 2013}}</ref> The latter two figures add to {{convert|11.6|kPa|Torr psi|sigfig=2|abbr=on}}, which is why many modern space suits do not use {{convert|20.7|kPa|Torr psi|sigfig=2|abbr=on}}, but {{convert|32.4|kPa|Torr psi|sigfig=2|abbr=on}} (this is a slight overcorrection, as alveolar partial pressures at sea level are slightly less than the former). In space suits that use 20.7&nbsp;kPa, the astronaut gets only 20.7&nbsp;kPa − 11.6&nbsp;kPa = {{convert|9.1|kPa|Torr psi|sigfig=2|abbr=on}} of oxygen, which is about the alveolar oxygen partial pressure attained at an altitude of {{convert|1860|m|ft|abbr=on}} above sea level. This is about 42% of normal partial pressure of oxygen at sea level, about the same as [[Cabin pressurization#Aircraft|pressure in a commercial passenger jet aircraft]], and is the realistic lower limit for safe ordinary space suit pressurization which allows reasonable capacity for work.

====Oxygen prebreathing====
{{see also|Decompression practice#Oxygen prebreathing}}
When space suits below a specific operating pressure are used from craft that are pressurized to normal [[atmospheric pressure]] (such as the [[Space Shuttle]]), this requires astronauts to "pre-breathe" (meaning pre-breathe pure oxygen for a period) before donning their suits and depressurizing in the air lock. This procedure purges the body of dissolved nitrogen, so as to avoid decompression sickness due to rapid depressurization from a nitrogen-containing atmosphere.<ref name="thomas" />

In the US space shuttle, cabin pressure was reduced from normal atmospheric to 70kPa (equivalent to an altitude of about 3000m) for 24 hours before EVA, and after donning the suit, a pre-breathing period of 45 minutes on pure oxygen before decompressing to the EMU working pressure of 30kPa. In the ISS there is no cabin pressure reduction, instead a 4-hour oxygen pre-breathe at normal cabin pressure is used to desaturate nitrogen to an acceptable level. US studies show that a rapid decompression from 101kPa to 55kPa has an acceptable risk, and Russian studies show that direct decompression from 101kPa to 40kPa after 30 minutes of oxygen pre-breathing, roughly the time required for pre-EVA suit checks, is acceptable.<ref name="thomas" />