And what matters in the atmosphere? The air above you.

You don't think it's all additive?

Sure, but the effects of air pressure are second order, if that.

Since this is soo off topic, this will be my last post on the subject
(we can carry on the conversation via email).

I apologize in advance for the non metric units.

If you're 14 feet under water (1 atmosphere of pressure, and you rise to
7 feet, the pressure on your body will be reduced by 50%. It takes you
18000 ft of air altitude change to achieve the same percentage. If you
are at the bottom of a 14 foot deep tank of water that gets lifted to
18000MSL, the pressure on your body is not reduced by 50% - not even
close to it.

Tony V