HOW loud is too loud? Sounds thought to cause only temporary hearing loss have destroyed nerve cells in the ears of mice. If the finding translates to humans, the laws that determine the noises workers can be exposed to may need to change.
Such regulations focus on preventing the destruction of hair cells in the inner ear, which causes a jump in someone鈥檚 hearing threshold. This threshold is 鈥渢he lowest level of sound you can detect鈥�, says of the Massachusetts Eye and Ear Infirmary in Boston. Now she and her colleagues have discovered that noises that aren鈥檛 loud enough to affect hearing thresholds can still cause permanent damage to ear cells.
鈥淣oises that aren鈥檛 loud enough to affect hearing thresholds can still cause permanent ear damage鈥�
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For 2 hours, Kujawa鈥檚 team 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. Under the gaze of a microscope, however, 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 (Journal of Neuroscience, ).
In people, these changes wouldn鈥檛 show up in hearing tests that measure thresholds, Kujawa says, as other nerves take up the slack. But such damage could explain more common hearing problems. 鈥淭he primary complaints that we hear from people is not 鈥業 can鈥檛 hear soft things鈥�, it鈥檚 鈥業 can鈥檛 understand speech amid noise, I have great difficulty understanding unless the speaker is standing right in front of me鈥�.鈥�
Neuroscientist at the University of Montpellier, France, says the implication is that 鈥渋f you are exposed during infancy or teenage years, you will lose hearing sooner when you are old鈥�.