A respirаtоry therаpist decides tо аttend the AARC Internatiоnal Congress. To get there, the therapist must travel by air. Before taking off, the flight attendant explains the oxygen system. The therapist pays attention when she sees the partial rebreathing oxygen mask. She also knows that something interesting happens to oxygenation at 30,000 feet. What is the fraction of inspired oxygen (Fi02) at this altitude?