According to the House Ear Institute, prolonged exposure to noise louder than 80db (alarm clock or busy street) is hazardous to your hearing health and may lead to permenant hearing damage.   They also report that the safe level for any 24 hour period of exposure to noise is 70db (restaurant noise).

So lets keep this in mind and enjoy our mp3 players, as well as our car radios, at safe and reasonable levels, and if needed, wear ear plugs at loud concerts.