Hazards: A Study Gauges the Risks for Ears With iPods - New York Times:iPod output depends partly on volume settings and partly on the music's intrinsic "recording/encoding levels". So device volume setting is only a part of what determines energy output. They might have done better to recommend both electronic level equalization and volume limitation together.
... The researchers, who are audiologists, concluded that the average young person could listen to a player at 70 percent of full volume for four and a half hours without much risk. They also said that if people used the earphones that come with the devices they could listen to music at an 80 percent level for 90 minutes a day without great risk.
But listening to the music full blast for just five minutes can affect hearing, they said...
Geriatric iPod users (age > 40, younger folk don't tolerate the dorky look) may wish to use noise canceling headphones. With noise canceling headphones a volume setting of 40-50% produces a good listening experience, with the default ear buds a comparable experience requires a volume setting of 70-80% in a standard office environment. Users who can tolerate them may use occlusive earphones to get a similar effect -- but beware otitis externa! Also, you won't hear the fire alarm ...
Closed ear phones are helpful, but probably don't deliver enough value to offset the associated bulk and inconvenience.