US health experts have urged further research into possible links between personal music players and potentially irreversible hearing loss.
Apple is facing a possible class action over iPod sound levels
The National Institutes of Health said new studies were needed into the effects of in-ear headphones used in music players like the Apple iPod.
The NIH was responding to calls by a US congressman into the possible long-term effects of loud music on hearing.
Apple is facing a lawsuit in California alleging the iPod can damage hearing.
John Kiel Patterson, of Louisiana, says his iPod is capable of generating more than 115 decibels, a dangerous noise level, and is not safe for prolonged use.
Democrat Edward Markey wrote to the NIH in January calling for the body to look at research into the growing use of personal music players.
"Sales of the devices have shattered all expectations. There is a very real need for research," he said.
"Kids are often more familiar with these products than parents, but they don't realise how harmful these products can be to hearing. It can lead to a lifelong ailment."
In a separate development, a poll of US youngsters suggested that as many of half of iPod users report symptoms of hearing loss.
The survey, by the American Speech-Language Hearing Association (ASHA), found that just 49% of US high school students reported no symptoms of hearing loss, compared to 63% of adults.
Symptoms include a need to turn up the volume on the TV or radio to hear broadcasts; asking someone to repeat part of a conversation; and ringing in the ears.
ASHA polled 301 high school students and 1,000 adults across the US.