Earbud headphones, like the ones typically used with iPods, project sound directly into the ear canal, while traditional earmuff-style headphones allow the sound to diffuse, the National Institutes of Health (NIH) said in a 14 February letter made available in Washington on Tuesday.
The proximity of the source of the sound to the ears can contribute to hearing loss, but "more research is required to determine if a particular type (of earphone) increases the risk," said James Battey, director of the National Institute on Deafness and Other Communication Disorders, in the NIH letter.
Representative Edward Markey, a Massachusetts Democrat, sent a letter on 26 January asking NIH to review research to determine whether portable music players are contributing to premature hearing loss as well as to recommend what people can do to prevent it from happening.
Markey said at a panel discussion on Tuesday that he planned to work with Representative Mike Ferguson, a New Jersey Republican, to encourage more study of a possible connection.
"Sales of the devices have shattered all expectations," Markey said. "There is a very real need for research."
Apple, which dominates the market for the devices, alone sold 14 million iPods during the Christmas holiday quarter and said in January that it had sold 42 million devices since October 2001, when the iPod was introduced.
A spokeswoman from Apple was not immediately available for comment.
Research into the phenomenon began in the early 1980s, soon after the 1979 introduction of portable radio and cassette players with headphones, like Sony Corp's Walkman.
"It is critically important to work to determine the best ways consumers can protect themselves from noise-induced hearing loss"
Representative Edward Markey,
Massachusetts Democrat, USA
The NIDCD during a 1990 conference concluded that "sounds of sufficient intensity and duration could damage the ear and result in temporary or permanent hearing loss at any age," Battey said.
He said research on the subject had surged in the past few years, after the introduction of portable MP3 players, which have maximum sound output levels comparable to the sound level of a jet engine.
Although recent studies have concluded that users should hold the volume of a music player to no higher than 60% of the maximum, Markey said more research was needed to help consumers.
"It is critically important to work to determine the best ways consumers can protect themselves from noise-induced hearing loss," he said.