Also, it may be when it comes to long term db levels, all dbs are not created equal.
For example, scientifically speaking, I might be able to see how 100db (pick a number) coming into your ears from ear canals over time might have a different effect than 100db coming from a speaker system, even if its in a car, etc.
Something to do with 1 of these 2 experiments, your eardrums have more airspace, etc., to deal with, and borrowing from material fatigue experiments, etc --> long story short is I wouldn't be shocked if it turns out that 100db x 1000 hours from IEMS is more damaging to hearing or ears than 100db x 1000 hours from speaker system.
I'm not sayin the above is true. But I will say this, as a gut feeling: if I had to choose between being exposed 1 hour to 140db via IEMS vs. 1 hour 140db via speakers, I'd choose the speakers...just seems to make sense that the speaker experiment *might* be less damaging --
the human body might be naturally more resilient to loud db's coming from far away, vs the more unnatural piped in dbs coming from iems.
I suppose counterargument is the above is irrelevant - whats important is the db level when it hits your eardrum...