Noise-Induced Hearing Loss

1. Noise-Induced Hearing Loss
Ozarks Technical Community College
HIS 125

2. Next to presbycusis, NIHL is the second-
most common cause of acquired hearing
loss
According to the National Institute for
Occupational Safety and Health (NIOSH),
NIHL is amongst the top ten work-related
injuries
NIHL is technically a noise-induced
permanent threshold shift (NIPTS)
Noise-Induced Hearing Loss (NIHL)

3. Two Types of NIPTS
Acoustic Trauma
Permanent hearing loss after a single noise
exposure
Most commonly due to impulse noise (i.e.
firearms)
Gradually-developing NIHL
Gradual hearing loss due to noise exposure
over an extended period of time (months to
years)

4. NIHL
Dependent on intensity, duration, and
type of noise exposure
The greater the intensity of the noise, the less
exposure time is required for damaging effects
to occur
Impulse noise has the potential to physically
break the hair cells with the potential for little
to no recovery, whereas a steady-state noise
(i.e. humming factory noise) may temporarily
stress the hair cells, which then recover after a
period of rest.

5. Loud noise results in damage to the
cochlea
OHCs are the 1
st
to be effected, followed by the
IHCs and eventually the nerve
Results in a SNHL with a “noise notch” on
the audiogram
Reduction in pure tone thresholds from 3 to 6
kHz, with recovery at 8 kHz

7. Why the notch at 4kHz?
The pinna effect and natural ear canal
resonance amplify sounds from 2-4 kHz
The acoustic reflex in the middle ear
reduces the amount of energy ≤500 Hz
that is transmitted to the cochlea
In humans, the frequency most damaged
by loud noise is ~ one-half to one octave
above the highest frequency contained in
the noise

8. Audiometric Variation
The noise-notch may occur at varying
frequencies depending on:
The frequency content of the noise
The size and shape of the individual’s ear canal
With continued noise exposure and
advancing age, the true noise notch will
disappear and the low frequencies will
begin to show hearing loss as well

9. Other Symptoms of NIHL
Tinnitus
Recruitment
Distortion of sounds (this may result in
poorer than expected word recognition
scores)

10. Individual Susceptibility
Some ears are “tougher” than others and more
resilient to the effects of loud noise
Factors that may influence susceptibility:
Middle ear function/acoustic reflex function
Protective effects of the efferent system
Prior conditioning noise exposure (a lesser noise
exposure may help protect or condition the ear to a
later, more significant noise exposure)
Genetic predisposition
Note: There is no way to predict individual susceptibility

11. Factors that exacerbate NIHL:
Simultaneous use of ototoxic drugs
Chemical exposure
Certain solvents used in industrial applications
Vibration