[rsofaer]

Submarines also have reduced partial pressure of oxygen, (18-19%). https://www.sciencedirect.com/science/article/pii/0378427489...

[JumpCrisscross]

> Submarines also have reduced partial pressure of oxygen, (18-19%)

The article defines high altitude at 1.96km, or 6,430 feet. That’s a less-than 16.5% effective oxygen atmosphere [1].

[1] https://www.higherpeak.com/altitudechart.html