Space exposure is the subjection of a human to the conditions of outer space, without protective clothing and beyond the Earth’s atmosphere in a vacuum.

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, 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.