You are on an airplane at an altitude of 20,000 feet where atmospheric pressure is 350 mm Hg. Air is pumped into the cabin for you to breath. What would the percent of oxygen in this are need to be to allow you to breath air with a partial pressure of oxygen of around the normal 150 mm Hg?
A) 10%
B) 20%
C) 40%
D) 100%
E) 150%

Question

Quizplus · Accepted Answer

The partial pressure of oxygen in normal air is around 21% x 760 mm Hg = 160 mm Hg. At an altitude of 20,000 feet, the atmospheric pressure is 350 mm Hg, so the partial pressure of oxygen would be approximately (160/760) x 350 = 73.7 mm Hg. To breathe air with a partial pressure of oxygen around the normal 150 mm Hg, the percentage of oxygen in the air would need to be (150/350) x 100 = 42.9%, or approximately 40%. Therefore, the best choice is C.

You Are on an Airplane at an Altitude of 20,000