Apple has sold more than 20 million iPods worldwide.

Some experts fear that the surge in sales of these and other portable music players that attach directly to the ears could lead to users developing hearing loss.

Music lovers turn the devices up to drown out what's going on around them and tend to listen to them for prolonged periods of time.

''It's a different level of use than we've seen in the past,'' says Robert Novak, director of clinical education in audiology at Purdue University in Indiana. ''It's becoming more of a full-day listening experience, as opposed to just when you're jogging.''

Noise-induced hearing loss happens any number of ways, from attending noisy concerts and clubs to using firearms or loud power tools and even recreational vehicles (snowmobiles and some motorcycles are among the offenders).

Today, doctors say many people also are wearing headphones, not just to enjoy music, but also to block out ambient noise on buses, trains or just the street.

The small, insertable earphones aren't as efficient at blocking background noise as the larger earphones that cover the whole ear. This forces users to turn up the volume to dangerously high levels in order to hear the music and successfully quiet background noise.

In a study published last year in the journal Ear and Hearing, Dr. Brian Fligor of Harvard Medical School looked at a variety of headphones and found that, on average, the smaller they were the higher their output levels at any given volume control setting.

At high volume, some portable players can reach 105 decibels. The sound of an aircraft taking off is 110 decibels. Since most CD and tape players can also reach this volume, the issue with MP3 players is the extended exposure to high volumes. The extensive battery life and vast memory space that holds thousands of songs mean that users can listen continuously for hours without needing to change CDs or recharge batteries.

"My biggest concern is the prolonged exposure," said Patricia Kricos, UF professor of audiology. "I see students using them all the time walking around campus."

Researchers at Boston Children's Hospital determined that listening to a portable music player with headphones at 60 percent of its potential volume for one hour a day is relatively safe.

Persuading college students to turn it down and turn it off might not be so easy.

"I don't think I listen to my iPod too loudly," said UF's Matt Turetzky, who's seeking a degree in business management. "But I don't think turning it up all the way is bad."

And he uses it, he said, on average about two hours a day.

Kricos recommends students gradually try to decrease the volume at which they typically listen to their iPods.

And they need to be able to recognize symptoms of hearing loss.

"I don't think students realize how damaging the effects can be and how hard it is to live with hearing loss," she said. "If your ears are ringing, that is a sign that you better turn it down."

By Lauren DeLucia