When is oxygen supplied at higher than atmospheric pressure in a pressure-demand system?

In a pressure-demand system, oxygen is supplied at higher than atmospheric pressure primarily at extremely high altitudes. This is essential because, at these altitudes, the ambient atmospheric pressure is significantly reduced, leading to a lower partial pressure of oxygen available for breathing.

The pressure-demand system compensates for this by providing oxygen under pressure, ensuring that the user receives a sufficient flow of oxygen, preventing hypoxia. This mechanism is crucial for pilots and passengers at high altitudes, where the thin atmosphere can quickly lead to reduced oxygen levels in the blood.

While other circumstances might involve the provision of oxygen (like maintenance procedures or possibly during exhalation), they do not require the oxygen to be supplied at a higher pressure compared to atmospheric levels as is necessary at extremely high altitudes.