It isn’t dangerous: it will just eventually damage your timpanic membrane, just as long-term exposure to loud noise in any environment would. *Extremely* loud noise (such as an explosion) can permanently damage the eardrums pretty much immediately, but headphones would never reach those sort of volumes.
People who go to a lot of loud concerts (or even nightclubs) over their life often develop hearing problems later in life. The same would be true if you blasted your ears with loud music for long periods.
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