Interestingly, that's higher than the ppO2 at sea level, which is about 3 psi, or at ~5K feet (standard airliner cabin pressure) where it's 2.5psi, so they were

Interestingly, that's higher than the ppO2 at sea level, which is about 3 psi, or at ~5K feet (standard airliner cabin pressure) where it's 2.5psi, so they were still running at almost double the amount of oxygen available normally.