Editing Outer space (section)

====Vacuum====
{{main|Uncontrolled decompression}}
The lack of pressure in space is the most immediate dangerous characteristic of space to humans. Pressure decreases above Earth, reaching a level at an altitude of around {{convert|19.14|km|mi|abbr=on}} that matches the [[vapor pressure of water]] at the [[Human body temperature|temperature of the human body]]. This pressure level is called the [[Armstrong line]], named after American physician [[Harry G. Armstrong]].<ref name=Tarver_et_al_2022/> At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule.{{sfn|Piantadosi|2003|pp=188–189}}

Out in space, sudden exposure of an unprotected human to very low [[Atmospheric pressure|pressure]], such as during a rapid decompression, can cause [[pulmonary barotrauma]]—a rupture of the lungs, due to the large pressure differential between inside and outside the chest.<ref name=Battisti_et_al_2022/> Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture.<ref name=krebs_pilmanis1996/> Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to [[Hypoxia (medical)|hypoxia]].<ref name=Busby_1967/>

As a consequence of rapid decompression, oxygen dissolved in the blood empties into the lungs to try to equalize the [[partial pressure]] gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes.<ref name=bmj286/> Blood and other body fluids boil when the pressure drops below {{convert|6.3|kPa|psi|0}}, and this condition is called [[ebullism]].<ref name=jramc157_1_85/> The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.{{sfn|Billings|1973|pp=1–34}}<ref name=landis20070807/>

Swelling and ebullism can be reduced by containment in a [[pressure suit]]. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as {{convert|2|kPa|psi|1}}.<ref name=am39_376/> Supplemental oxygen is needed at {{Convert|8|km|mi|0|abbr=on}} to provide enough oxygen for breathing and to prevent water loss, while above {{Convert|20|km|mi|abbr=on}} pressure suits are essential to prevent ebullism.{{sfn|Ellery|2000|p=68}} Most space suits use around {{convert|30|-|39|kPa|psi|0}} of pure oxygen, about the same as the partial pressure of oxygen at the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause [[decompression sickness]] and [[air embolism|gas embolisms]] if not managed.{{sfn|Davis|Johnson|Stepanek|2008|pp=270–271}}