Its barometric altimeter tracks changes in pressure to pinpoint users' altitude, and they even can use it to plot barometric pressure over time, which can help keep an eye on changing weather conditions.

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At Sunday's jump altitude, the air pressure is less than 2% of what it is at sea level, and it is impossible to breathe without an oxygen supply.

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The original Mercury spacesuits were a modified version of pressure suits that pilots wore in high-altitude jet aircraft.

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These trekking groups are often on a tight itinerary and may ignore the early symptoms of altitude sickness - headaches, loss of appetite - or may simply hide them because of peer pressure.

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Although jets are pressurized so that the cabin environment resembles an altitude of around 8, 000 feet, some people report that flying (possibly from the changes in pressure and resulting decrease in oxygen concentration) causes symptoms such as headache, fatigue and nausea.

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