If I recall correctly the maximum Noise Reduction Rating (NRR) for earplugs and earmuffs is around 30db. You can combine the two for a slight increase in hearing protection but you still hit a limit because of bone vibration.
Is there PPE out there to go even further beyond this? Where would it be commonly used?
If you blasted the sound outside of the headphones into the cabin it would decrease that for you and increase it for everyone else. But since you don’t walk around with two giant loudspeakers strapped to your passive noise canceling (isolation) headphones, yes the pressure is added to your ears and your ears only. You just can’t perceive it, outside of the headaches and fatigue over time…