Listening to loud music with earphones on portable digital music players such as iPods for more than 90 minutes a day can damage your hearing, a new U.S. study suggests.

The study indicates a typical person can safely listen to an iPod for 4.6 hours per day at 70 per cent volume using stock earphones, said Cory Portnuff, a doctoral researcher at the University of Colorado, Boulder and co-author of the study.

"Damage to hearing occurs when a person is exposed to loud sounds over time," he said Thursday. "The risk of hearing loss increases as sound is played louder and louder for long durations, so knowing the levels one is listening to music at, and for how long, is extremely important."

The study of 100 doctoral students found that people who listened to music at 80 per cent of volume capacity should stick to under 90 minutes a day, said Brian Fligor, the study's co-author and an audiologist at Children's Hospital Boston and Harvard Medical School.

"If a person exceeds that on one particular day and happens not to use their headphones for the rest of the week, they're at no higher risk," Fligor told Reuters. "I'm talking about someone who's exceeding 80 per cent for 90 minutes day after day, month after month, for years."

The study found no problems for people who listened to music at 10 per cent to 50 per cent of maximum volume for extended periods. However, it found that anyone who listened at 100 per cent for more than five minutes faced the risk of hearing loss.

Player brand, genre not an issue

The researchers also found no significant difference in sound levels between five of the most popular genres of popular music they tested, including rock, rhythm and blues, dance, Top 40 and country.

In a second study, Fligor and Terri Ives of Pennsylvania's College of Optometry's School of Audiology found that in-ear earphones, such as the ubiquitous white iPod earphones that broadcast sound directly into the ear cavity, are no more dangerous than headphones that are placed over the ears.

But the study also found people are much more likely to raise the volume to risky levels if they are listening to their music amidst noisy surroundings.

The studies were presented Thursday at a conference on noise-induced hearing loss in children in the Cincinnati area.