Above what altitude should a diluter-demand regulator provide only oxygen?

The diluter-demand regulator is designed to provide a mixture of oxygen and ambient air at lower altitudes to help maintain the physiological needs of the crew while conserving oxygen supply. However, as altitude increases, the partial pressure of oxygen in the atmosphere decreases, which can lead to hypoxia if not managed properly.

Above thirty-four thousand feet mean sea level, the atmospheric pressure becomes so low that the availability of sufficient ambient air to mix with the oxygen from the regulator is no longer effective in meeting oxygen requirements. At this altitude, the diluter-demand regulator will automatically switch to delivering 100% oxygen. This switch ensures that the pilot and crew receive adequate oxygen to prevent hypoxia, which can impair cognitive and physical performance. Thus, the correct response reflects a critical safety threshold for oxygen delivery in high-altitude operations.