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Even Less Loud Sounds can Damage Ears: Study

by VR Sreeraman on  November 15, 2009 at 9:54 AM Research News   - G J E 4
 Even Less Loud Sounds can Damage Ears: Study
Sounds, which are comparatively less loud than damaging noise, can take a toll on a person's hearing ability, a study on mice has found.

In the research, researchers found that sounds thought to cause only temporary hearing loss, destroyed nerve cells in the ears of mice.
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And if the finding translates to humans, the laws that determine the noise workers can be exposed to may need to change.

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Such regulations focus on preventing the destruction of hair cells in the inner ear, which causes a jump in someone's hearing threshold.

The threshold is "the lowest level of sound you can detect", says Sharon Kujawa of the Massachusetts Eye and Ear Infirmary in Boston.

Now the researchers have discovered that noises that aren't loud enough to affect hearing thresholds can still cause permanent damage to ear cells.

For two hours, researchers exposed mice to a 100-decibel blare, roughly equivalent to a motorcycle engine or approaching subway train.

Several tests indicated that this ruckus caused no long-lasting changes in hearing threshold.

However, when observed through a microscope, damage was seen to the part of hair cells that transmits sound via chemical interactions with nearby nerves.

A year later, the damage had seemingly spread to nerves that transmit sound to the brain.

Kujawa said that in people these changes wouldn't show up in hearing tests that measure thresholds as other nerves take up the slack.

But such damage could explain more common hearing problems.

"The primary complaints that we hear from people is not 'I can't hear soft things', it's 'I can't understand speech amid noise, I have great difficulty understanding unless the speaker is standing right in front of me,'" New Scientist quoted her as saying.

Neuroscientist Jean-Luc Puel at the University of Montpellier, France, says the implication is that "if you are exposed during infancy or teenage years, you will lose hearing sooner when you are old".

The study has been published in the Journal of Neuroscience.

Source: ANI
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