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Altitude decompression illness (DCI) is generally considered to be a risk at altitudes in excess of 18,000 ft. UK military aircrew are therefore not routinely exposed to altitudes in excess of this, however, there are circumstances such as loss of cabin pressure, parachute operations, and high cabin altitudes in future aircraft, when exposure to altitudes in excess of 18, 000 ft may be necessary. A series of experiments were carried out at the DERA center for Human Sciences to investigate the risk of venous gas emboli (VGE) and DCI symptoms at altitudes up to 35,000 ft. Subjects were exposed, for a maximum of four hours, to (1) simulated altitudes between 20,500 ft and 25,000 ft breathing an oxygen/nitrogen gas mixture, (2) 25,000 ft breathing 100% oxygen with and without one hour of prior denitrogenation and (3) simulated altitudes up to 35,000 ft with one hour prior denitrogenation. It was concluded that VGE formation will occur at cabin altitudes that will be encountered by aircrew of future agile aircraft although only 7% of subjects developed symptoms.