Above 3000 metres (10,000 feet) - ambient pressure 69.7kPa, about 14.6kPa partial pressure of oxygen – enough hypoxic stimulation to cause increased ventilation Above 3700 metres (12,000 feet) - 64.4kPa, about 13.52kPa PO2 – first irritability appears Above 5500 metres (18,000 feet) - 50.6kPa, about 10.6kPa PO2 – severe symptoms Above 7600 metres (25,000 feet) - ambient pressure 37.6kPa absolute, 7.9kPa partial pressure of oxygen – consciousness lost While breathing pure oxygen at ambient pressure, from an oxygen cylinder or other source, the maximum altitude a human can tolerate while their body is at atmospheric pressure is 13,700 metres (45,000 feet), , where atmospheric pressure is about 14.7kPa. This is a function of the partial pressure of oxygen in the breathing gas, and is also dependent on level of exertion which affects the oxygen requirements of metabolism, cardiovascular fitness, and acclimatization to altitude which affects the available hemoglobin and can vary significantly between individuals. Signs and symptoms Cyanosis Headache Decreased reaction time, disorientation, and uncoordinated movement. Impaired judgment, confusion, memory loss and cognitive problems. Euphoria or dissociation Visual impairment Lightheaded or dizzy sensation, vertigo Fatigue, Drowsiness or tiredness Shortness of breath Palpitations may occurin the initial phases. Later, the heart rate may reduce significantly degree. In severe cases, abnormal heart rhythms may develop. Nausea and vomiting Initially raised blood pressure followed by lowered blood pressure as the condition progresses. Severe hypoxia can cause loss of consciousness, seizures or convulsions, coma and eventually death. Breathing rate may slow down and become shallow and the pupils may not respond to light. Tingling in fingers and toes Numbness Treatment Generalized hypoxia is an effect of a lack of oxygen, and in many cases of a one-time event can be reversed simply by eliminating the lack. Where there is no underlying pathology, provision of oxygen at normobaric partial pressure (about 0.21 bar) is usually sufficient to reverse minor symptoms. Where there is a pathology causing the hypoxia, treatment of the underlying pathology is often effective. Oxygen therapy and hyperbaric medicine can improve saturation of oxygen in the blood which will stop hypoxia if it is caused by hypoxemia, i.e. low levels of oxygen in the blood. Artificial ventilation may be required where the person is unable to breathe sufficiently unaided. Continuous positive airway pressure (CPAP) mask may be used to treat sleep apnea. Bilevel positive airway pressure (BIPAP) may be used to treat chronic