Noise-induced Hearing Loss (NIHL)

How noise damages the ear: The mechanism of noise-induced hearing loss.

Our ears were designed to process naturally-occurring sounds, and they are beautifully adapted to handle that task. They are able to detect sounds of intensities that vary across many orders of magnitude, and to meaningfully transmit those signals to our brains. But they are not well equipped to deal with the high noise levels that are common today, because such loud sounds occur only rarely in nature.

structure of the ear
Image courtesy of NASA

The ear is a complex structure, processing sound through several stages in the outer, middle, and inner ear. Although the eardrum may sometimes be ruptured by severe noise (acoustic trauma) or pressure changes, the part that is most vulnerable to damage by noise lies more deeply, in the inner ear, where the final processing takes place before the sound is converted into nerve impulses that are transmitted to the brain.

The prominent structure in the inner ear is the spiral-shaped chochlea, which is a fluid-filled tube lined with delicate, microscopic hair cells that pick up the vibrations caused by sound waves. When they are overworked by too much exposure to loud sounds, the hair cells become metabolically exhausted and can temporarily lose their function. Fortunately, they are able to recover from the auditory fatigue caused by too much noise, but if overexposure is too long or too frequent, they can't cope, and they die. There is no pain or bleeding when this occurs.

healthy hair cells
Image courtesy of NASA
Healthy hair cells

damaged hair cells
Image courtesy of NASA
Damaged hair cells

There are about 15,000 of these hair cells in the chochlea, and when one dies, it is irreplaceable, and that part of your hearing sensitivity is gone forever.

You can think of the hair cells like individual blades of grass in a section of lawn. You can walk across the lawn, stepping on the blades of grass, and they will recover, but if many people continually walk across the same area, the grass will become sparse and may eventually disappear as a path is worn. That is analogous to what happens to hair cells when the "traffic" from noise is too heavy or too frequent.

Hair cells don't "toughen up" with exposure to high noise levels and become less vulnerable to damage. The inner ear is not a muscle which strengthens with use, but a delicate instrument, relying on the proper functioning of its individual parts in order to work. If you want to keep your hearing in the best condition, be kind to your ears, and protect them from hearing damage caused by noise abuse.

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