The study of 100 doctoral students concluded that people who listened to music at 80 percent of volume capacity, at which point the sound is considered loud, should stick to under 90 minutes a day.

"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," study author Brian Fligor told Reuters. "I'm talking about someone who's exceeding 80 percent for 90 minutes day after day, month after month, for years."

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

The findings of the study, co-authored by doctoral candidate Cory Portunff, applies to children and adults. The researchers do not know if children are more susceptible than adults.

The scientists found no differences in sound levels between brands of digital music players or between the genres of music tested which ranged from rock, R&B, country, to dance.

Fligor, an audiologist at the Children's Hospital of Boston and faculty of Harvard Medical School, said people who consistently listen to high levels of volume don't realize that hearing loss can take up to 10 years to show up.

"I worry about the teen-ager who's going to be 23, 24, 25 years old and has a measurable noise-induced hearing loss and now has another 60-something years to live with his hearing which is only going to get worse," said Fligor, who will present his study on Thursday to a conference in Cincinnati.

Fligor will also present the findings of another study, co-authored by Terri Ives of Pennsylvania's College of Optometry's School of Audiology, that found in-ear earphones, which broadcast sound directly into the ears, are no more dangerous than headphones that are placed over the ears.

Both studies are being delivered to a "Noise-Induced Hearing Loss in Children in Work and Play" meeting.