Noise-Induced Hearing Loss (NIHL)
Of the roughly 40 million Americans suffering from hearing loss, 10 million
can be attributed to noise-induced hearing loss (NIHL). NIHL can be caused by a one-time exposure to loud
sound as well as by repeated exposure to sounds at various loudness levels over an extended period of time.
OSHA requires a Hearing
Conservation program and training when workers are exposed to noise over 85 decebels.
Damage happens to the
microscopic hair cells found inside the cochlea. These cells respond to mechanical sound vibrations by sending
an electrical signal to the auditory nerve. Different groups of hair cells are responsible for different
frequencies (rate of vibrations).
The healthy human ear can hear
frequencies ranging from 20Hz to 20,000 Hz. Over time, the hair cell's hair-like stereocilia may get damaged or
broken. If enough of them are damaged, hearing loss results. The high frequency area of the cochlea is often
damaged by loud sound.
Sound pressure is measured in
decibels (dB). Like a temperature scale, the decibel scale goes below zero. The average person can hear sounds
down to about 0 dB, the level of rustling leaves. Some people with very good hearing can hear sounds down to -15
If a sound reaches 85 dB or
stronger, it can cause permanent damage to your hearing. The amount of time you listen to a sound affects how
much damage it will cause. The quieter the sound, the longer you can listen to it safely. If the sound is very
quiet, it will not cause damage even if you listen to it for a very long time; however, exposure to some common
sounds can cause permanent damage.
Extended exposure, noises that
reach a decibel level of 85 can cause permanent damage to the hair cells in the inner ear, leading to hearing
loss. Many common sounds may be louder than you think…
· A typical conversation occurs at
60 dB - not loud enough to cause damage.
· A bulldozer that is idling (note
that this is idling, not actively bulldozing) is loud enough at 85 dB that it can cause permanent damage after only
1 work day (8 hours).
· When listening to music on
earphones at a standard volume level 5, the sound generated reaches a level of 100 dB, loud enough to cause
permanent damage after just 15 minutes per day!
· A clap of thunder from a nearby
storm (120 dB) or a gunshot (140-190 dB, depending on weapon), can both cause immediate damage.
In fact, noise is probably the most common occupational hazard facing people today. It is estimated that as many as
30 million Americans are exposed to potentially harmful sounds at work.
outside of work, many people participate in recreational activities that can produce harmful noise (musical
concerts, use of power tools, etc.). Sixty million Americans own firearms, and many people do not use
appropriate hearing protection devices.
Exposure Time Guidelines
Accepted standards for
recommended permissible exposure time for continuous time weighted average noise, according to NIOSH and CDC,
2002. For every 3 dBs over 85dB, the permissible exposure time before possible damage can occur is cut in
Permissible Exposure Time (PET)
3.75 min (<4 min)
1.875 min (<2 min)
.9375 min (< 1 min)
.46875 (~30 sec)