The reasoning: Dalton, Boyle and Henry
- Dalton: PiO₂ = FiO₂ × (PB − 47). FiO₂ is 0.21 everywhere; PB falls ~exponentially with height.
- Boyle: P × V constant — gas volumes in closed spaces expand on ascent (sinuses, middle ear, pneumothorax, bowel) and shrink on descent.
- Henry: dissolved gas ∝ partial pressure — nitrogen loads tissues at depth and leaves on ascent.