by Crystal Phend

iPod users beware. Listening to an MP3 player for even an hour can induce short-term hearing loss, researchers affirmed.

Compared with controls, hearing deterioration among MP3 users was 3.97 to 4.40 times more likely when listening to pop/rock music through earbud headphones at different output levels, found Hannah Keppler, MS, of Ghent University in Ghent, Belgium, and colleagues.

Listening to an iPod with over-the-ear headphones also boosted the odds of a short-term decline in hearing threshold by 2.18- to 3.87-fold (P<0.05 to P<0.001), they reported in the June issue of Archives of Otolaryngology-Head & Neck Surgery.

By an objective measure, damage to the “hair cells” in the inner ear was 4.70 times more likely after an hour of listening with earbuds and 5.96 times more with headphones than among controls (both P<0.001).

Sound sensitivity appeared to recover to baseline levels between listening sessions.

The researchers cautioned that these short-term results couldn’t determine permanent effects on hearing.

“But, considering the reduction in hearing sensitivity after listening to a personal music player, these devices are potentially harmful,” they wrote.

The standard iPod earbud headphones can put out 100.0 to 110.5 decibels of sound, which exceeds the 75-decibel threshold for risk of noise-induced hearing loss, leading to widespread concern about effects on hearing, Keppler’s group noted.

Damaging effects have been largely assumed, since prevalence of hearing loss in young adults hasn’t risen since 1985, when Sony Walkman cassette players were all the rage, the researchers said.

But it may be that intensive exposure to music doesn’t permanently affect hearing because it’s usually limited to five or 10 years in a lifetime, they suggested.

Or the Walkman and iPod generations may not have gotten old enough yet for the real long-term effects to show up, they added.

In addition to 28 controls, their study included 21 participants with normal pre-exposure hearing who sat down for six different one-hour sessions listening to Belgium’s most popular pop/rock songs on an iPod Nano with at least 48 hours between successive sessions.

Four sessions used preset gain setting of 50% or 75% with the stock iPod earbuds or over-the-ear Sennheiser headphones.

Another two sessions with both headphone styles allowed participants to set the gain at “a loud but comfortable setting,” which was 90% or 100% for all but six of the participants.

At these gain settings from 50% to 100% the earbuds put out 76.87 to 102.56 decibels while the headphones were typically 5.55 decibels lower at every setting. However, type of headphone appeared to have little impact on temporary hearing loss.

Immediately after listening sessions, audiometric hearing threshold tests — which rely on subjective response to sounds — showed a significant 1.12-dB drop in sensitivity compared with pre-exposure measurements at the 0.25 kHz exposure level and a 1.17-dB deterioration at 8.0 kHz (both P<0.05). No other frequencies showed significant main effects.

Otoacoustic emissions, indicating the motile activity of hair cells in the ear, were also measured as an objective barometer of cochlear integrity.

These emission amplitudes declined by 0.47 decibels at 2.0 kHz (P<0.05) and by 0.70 decibels at 2.8 kHz (P<0.001).

Higher volume appeared to increase the effect on short-term hearing loss by this measure at 2.8 kHz and 4.0 kHz (both P<0.01).

The difference between these two types of measures may be partly accounted for by the fact that otoacoustic emissions are thought to be an earlier indicator of damage as it detects inner ear changes before hearing is affected, the researchers said.

They noted, though, that these two tests were done in a fixed order and thus may have reflected different ponts in the recovery process.

The shifts in hearing thresholds and emissions didn’t appear to increase with subsequent listening sessions.

The study may have underestimated the effect if output levels are higher for iPods made for markets outside Europe, since France has a 100-decibel limit on MP3 player output, the researchers suggested.

Also, different types of MP3 players may have somewhat different sound output levels, they warned.

Crystal Phend is a MedPage Today Senior Staff Writer.

Originally published in MedPage Today. Visit MedPageToday.com for more preventive care news.