Explanation and history
The key concerns for a human without protective clothing beyond Earth’s atmosphere are the following, listed roughly in the descending order of mortal significance: ebullism, hypoxia, hypocapnia, decompression sickness, extreme temperature variations and cellular mutation and destruction from high energy photons and (sub-atomic) particles.
For the effect of rapid decompression to vacuum conditions, see the main article at Uncontrolled decompression.
Ebullism, hypoxia, hypocapnia and decompression sickness
Ebullism, the formation of bubbles in body fluids due to reduced ambient pressure, is the most severe component of the experience. Technically, ebullism is considered to begin at an elevation of around 19 kilometres (12 mi) or pressures less than 6.3 kPa (47 mm Hg), known as the Armstrong Limit. Experiments with other animals have revealed an array of symptoms that could also apply to humans. The least severe of these is the freezing of bodily secretions due to evaporative cooling. But severe symptoms such as loss of oxygen in tissue (anoxia) and multiplicative increase of body volume occur within 10 seconds, followed by circulatory failure and flaccid paralysis in about 30 seconds. The lungs also collapse (atelectasis) in this process, but will continue to release water vapour leading to cooling and ice formation in the respiratory tract.
A rough estimate is that a human will have about 90 seconds to be recompressed, after which death may be unavoidable. Unconsciousness is likely to occur within 14 seconds, primarily due to the much lower pressure outside the body causing rapid de-oxygenation of the blood (decompression sickness), which is less severe in space than in diving. Meanwhile, reduction of blood carbon dioxide levels (hypocapnia) can alter the blood pH and indirectly contribute to nervous system malfunctions. If the person tries to hold their breath during decompression, the lungs may rupture internally.
Few humans have experienced these four conditions. Joseph Kittinger experienced localised ebullism during a 31 kilometres (19 mi) ascent in a helium-driven gondola. His right-hand glove failed to pressurise and his hand expanded to roughly twice its normal volume accompanied by disabling pain. His hand took about 3 hours to recover after his return to the ground. Two other people were decompressed accidentally during space mission training programs on the ground, but both incidents were less than 5 minutes in duration, and both victims survived. International Space Station and Space Shuttle astronauts regularly work in Extravehicular Mobility Units (EMUs or space suits) that are at pressures less than 30% of the spacecraft to facilitate mobility, without experiencing noticeable decompression sickness.
The only known humans to have died of space exposure are the three crew members of the Viktor Patsayev. During the re-entry on June 30, 1971, the ship's depressurization resulted in the death of the entire crew.
Decompression is a serious concern during the 
Extreme temperature variations
Extreme temperature variations are a problem in space, because heat exchange occurs primarily via infrared radiation. While the absence of convection and conduction causes an insulating effect preventing rapid dissipation of body heat, localized heating can occur if exposed to sunlight at distances comparable to the Earth–Sun distance, and radiative loss of body heat can approach 1,000 watts in a worst-case scenario, given a skin temperature of 37 °C, and a body surface area of 2 square meters.
In a vacuum water vapor would rapidly evaporate from exposed areas such as the lungs, cornea of the eye, and mouth, cooling the body. Rapid evaporative cooling of the skin would create frost, particularly in the mouth, but this does not represent a significant hazard (relative to ebullism, etc.): the effective black-body radiation temperature throughout most of space is very close to absolute zero, but a vacuum does not support transfer of heat by convection or conduction; so the main temperature regulation concern is excess naturally generated body heat.
Cellular mutation and destruction from high energy photons and (sub-atomic) particles
A more severe long-term effect would be the direct exposure to high energy photons (ultraviolet, X-ray, and gamma) and energized subatomic particles (primarily protons). These can permanently denature DNA and other cellular molecules through atomic and nuclear interactions. Prolonged exposure and the ability of X and gamma photons to penetrate the entire body may cause death from organ failure, while even short-term exposure may cause cancer.
In science fiction
Spacing is a staple of science fiction, where it usually occurs as a method of execution (or other sort of killing) by vacuum exposure in space—usually accomplished by ejecting the subject through the airlock of a spacecraft or space station without a space suit. Spacing is sometimes used as a means of dispatching enemies, usually by luring or herding the target(s) into an airlock, hangar or cargo bay with an exterior hatch and then flushing them out into space, or opportunistically double-opening an airlock—or even blowing out a window or hull panel—that happens to be near the target, with similar results. The primary cause of death would be asphyxia. Spacing is how actress Sigourney Weaver disposes of the creatures in the 1979 science-fiction classic Alien and its 1986 sequel Aliens.
Many works of fiction show or describe the effects of exposure to the vacuum of outer space in unrealistic ways, such as showing the victim exploding.
- Pilmanis, Andrew; William Sears (December 2003). "Physiological hazards of flight at high altitude". The Lancet 362: s16–s17.
- Billings, Charles E. (1973). "Chapter 1) Barometric Pressure". In James F.; West, Vita R. Bioastronautics Data Book (Second ed.). NASA. pp. 2–5. NASA SP-3006. Retrieved 2012-09-23. PDF
- NASA Ask an Astronomer
- Higgins, Matt (May 24, 2008). "20-Year Journey for 15-Minute Fall".
- "Skydive from the Stratosphere", NOVA Online, Public Broadcasting Service(PBS). November 2000. Retrieved 2012-09-23
- Stewart, L. et al. (2007), doi 10.1016/j.jemermed.2006.05.031
- "Science: Triumph and Tragedy of Soyuz 11". Time Magazine (Time Inc.). July 12, 1971. Retrieved 2012-09-23. (subscription required)
- Conkin, Johnny (January 2001), "Evidence-Based Approach to the Analysis of Serious Decompression Sickness With Application to EVA Astronauts" NASA TP-2001-210196. Retrieved 2012-09-23. PDF
- Jordan, Nicole C.; Saleh, J.H.; Newman, D.J. (2005), "The Extravehicular Mobility Unit: case study in requirements evolution". doi 10.1109/RE.2005.69. Requirements Engineering, 2005. Proceedings.13th IEEE International Conference, pp.434-438. Retrieved on 2012-09-23 (subscription required)
- Jordan, Nicole C. et al. (2006). "The extravehicular mobility unit: A review of environment, requirements, and design changes in the US spacesuit". Acta Astronautica 59 (12): 1135–1145.
- Gorguinpour, Camron et. al (2001), LPI "Advanced Two-System Space Suit". University of California, Berkeley CB-1106. Retrieved 2012-09-23. PDF
- for reference, the atmospheric pressure at sea level is 101.4 kPa, equal to 14.7 psi – Britannica
- Boynton, W. V. et al. (2004), doi 10.1023/B:SPAC.0000021007.76126.15 (subscription required)