By Sunday, she had improved significantly to 33% -- just 12 percentage points shy of 21%, the level of oxygen in air around us.

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As the oxygen level fell, though, something else rose: the level of urea.

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Inside the lodges, trekkers rest up ahead of their final climb, conserving their strength and yet still burning energy just trying to breathe and function at such an altitude -- at 4, 800m, oxygen levels are only about 55% of those at sea level.

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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 researchers did note that many of the cases of lightheadedness could be due to the relatively low oxygen content of cabin air-roughly equal to 8, 000 feet above sea level-or to passengers experiencing dehydration during long or multiple flights.

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