Barometric pressure at 10,000 feet deep in the sea is

The approximate barometric at sea level, the barometric pressure is 760 mm Hg; at 10,000 feet, it is only 523 mm Hg; and at 50,000 feet, it is 87 mm Hg. This decrease in barometric pressure is the basic cause of all the hypoxia problems in high-altitude physiology because, as the barometric pressure decreases, the atmospheric oxygen paial pressure (PO2) decreases propoionately, remaining at all times slightly less than 21 percent of the total barometric pressure; At sea level, PO2 is about 159 mm Hg, but at 50,000 feet, PO2 is only 18 mm Hg.Ref: Guyton; 13th edition; Chapter 44; Ation, High Altitude, and Space Physiology