are quite variable. By contrast, suddenly breathing pure inert gas causes oxygen levels in the blood to fall precipitously, and may lead to unconsciousness in only a few breaths, with no symptoms at all. Some animal species are better equipped than humans to detect hypoxia, and these species are more uncomfortable in low-oxygen environments that result from inert gas exposure; however, the experience is still less aversive than CO2 exposure. Physiology A typical human breathes between 12 and 20 times per minute at a rate influenced primarily by carbon dioxide concentration, and thus pH, in the blood. With each breath, a volume of about 0.6 litres is exchanged from an active lung volume of about three litres. The normal composition of the Earth's 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. Unconsciousness in cases of accidental asphyxia can occur within one minute. Loss of consciousness results from critical hypoxia, when arterial oxygen saturation is less than 60%. "At oxygen concentrations [in air] of 4 to 6%, there is loss of consciousness in 40 seconds and death within a few minutes". At an altitude over , where the ambient oxygen concentration is equivalent to a concentration of 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 and euphoria, and some become unconscious without warning. Loss of consciousness may be accompanied by convulsions and is followed by cyanosis and cardiac arrest. About seven minutes of oxygen deprivation causes death of the brainstem. In a 1963 study by RAF Institute of Aviation Medicine, subjects were asked to hyperventilate in a nitrogen atmosphere. Among the results: The study did not report how much discomfort the subjects felt. Animal slaughter Relation to controlled atmosphere killing Controlled atmosphere killing (CAK) or controlled atmosphere stunning (CAS) is a method for slaughtering animals such as chickens or cane toads by placing the animals in a container in which the atmosphere lacks oxygen and consists of an asphyxiant gas (one or