Another bit of geeky science to add to Cosmom's input: One interesting thing about the neural connections of the ear is that the number of sensory cells processing the info from the cochlea is extremely small. We are talking in the thousands range, which on the scale of neurologic systems is infinitesimal (compare with millions of retinal sensors for vision). We lose some in time automatically, and they do not regenerate. There is high frequency hearing loss with age in everyone (fancily dubbed presbyacusis, ie old folks hearing), but this is certainly accelerated by exposure to loud noise. Chronic exposure is the worst. The time of exposure that is safe depends on the level. At about 90dB you are probably OK for 8 hours a day. At 100 dB that goes down to only 2 hours. Short bursts can be dangerous too, but only above 120dB or so. The numbers above are those used by the Occupational Safety and Health Administration in the US.
Low frequencies are relatively protected, for some reason most loss occurs around 4kHz like someone pointed out, and then the extreme highs, above ~15kHz.
So bottom line is: reasonable levels are fine, if they mimic say the sound in a classical music concert hall. Note that when you are in a concert hall you can easily hear that annoying dude 3 rows back shuffling his candy wrapping, so if the music you listen to is loud enough to prevent hearing similar sounds, it might be too loud.
Put another way, your lawn mower probably puts out about 90 dB, so if your headphones are as loud you should start counting hours of listening.
Hope it helps.
