The National Institutes of Health (NIH) today said additional research is required to determine whether portable music players--such as Apple's iPod--increase the risk of hearing loss. The statement was made in response to a U.S. lawmaker's request for a review of the issue. Representative Edward Markey, a Massachusetts Democrat, sent a letter on January 26th requesting that NIH 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, according to a report from Reuters. The NIH said in a letter today that Earbud headphones, like those typically used iPods, project sound directly into the ear canal, while traditional earmuff-style headphones allow the sound to diffuse. "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.