From the journal of Aviation Technology and Engineering:
Effects of mild hypoxia on pilot performances at general aviation altitudes, the authors found that perceptual-motor performance at pressure-altitude equivalents of 7,000 and 12,000 feet in a hypobaric chamber had significantly slower response times at both altitudes as compared to sea level (Nesthus, Rush, & Wreggit, 1997).
I have personally experienced hypoxia from flying at 9,500 density altitude for an hour.
I live at sea level so I am not acclimated to high altitudes.
The FAR is:
§ 91.211 Supplemental oxygen.
(a)
General. No
person may operate a
civil aircraft of U.S. registry -
(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;
(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire
flight time at those altitudes; and
(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the
aircraft is provided with supplemental oxygen.