Physiology
A typical human breathes between 12 and 20 times per minute at a rate primarily influenced by carbon dioxide concentration, and thus pH, in the blood. With each breath, a volume of about 0.6 liters is exchanged from an active lung volume (tidal volume + functional residual capacity) of about 3 liters. Normal Earth atmosphere is about 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, and other gases. After just two or three breaths of nitrogen, the oxygen concentration in the lungs would be low enough for some oxygen already in the bloodstream to exchange back to the lungs and be eliminated by exhalation. Crude simulation of oxygen transport through the lungs and bloodstream suggests that the partial pressure of oxygen in arterial blood would be about 50% of saturation 1 minute after switching gases and would reach zero within 3 minutes.
Unconsciousness in cases of accidental asphyxia can occur within 1 minute. Loss of consciousness results from critical hypoxia, when arterial oxygen saturation is less than 60%. "At oxygen concentrations of 4 to 6%, there is loss of consciousness in 40 seconds and death within a few minutes". As this procedure provides an atmosphere completely devoid of oxygen, the sequence of effects should be expected to occur even more quickly. At an altitude over 43,000 ft (13,000 m), where the ambient oxygen concentration is equivalent to 3.6% at sea level, an average individual can perform flying duties efficiently for only 9 to 12 seconds without oxygen supplementation. The US Air Force trains air crews to recognize their individual subjective signs of approaching hypoxia. Some individuals experience headache, dizziness, fatigue, nausea, or euphoria, but some become unconscious without warning. Equivalent training is unlikely for a condemned individual, making unconsciousness without warning probable, although as much as a 30 second warning is possible.
Loss of consciousness may be accompanied by convulsions and is followed by cyanosis and cardiac arrest. About 7 minutes of oxygen deprivation causes death of the cerebral cortex and presumably of the medulla oblongata, which controls breathing and heart action.
Read more about this topic: Nitrogen Asphyxiation
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