The massive popularity of iPods and other portable music players could mean many more people will develop hearing loss, according to experts.
If the volume through headphones is too high, there is a real risk of permanent damage to hearing, they say.
Sydney's National Acoustic Laboratories found a quarter of personal music system users in a random sample listened to music at dangerous volumes.
Millions of people now own MP3 players and Apple has sold more than 20 million iPods.
A recent study by the Royal National Institute for Deaf People (RNID) found 39% of 18 to 24-year-olds listened to personal music players for at least an hour every day and 42% admitted they thought they had the volume too high, according to the BBC News website.
The RNID regards 80 decibels as the level at which hearing is threatened - 20 less than a pneumatic drill.
Some MP3 players can reach 105 decibels. EU iPods have a sound limiter to comply with noise safety levels; however, sometimes users hack through this in order to listen to it louder.
The RNID said it was possible that any rise in popularity of personal music players might lead to more cases of hearing loss in the future.
Graham Frost, chairman of the British Society of Audiology, said the risk of damage increased with noise level and duration of use of personal music systems.
He said it could take months or years for that become apparent to the individual.
