Noise-Induced Hearing Loss

Ozarks Technical Community CollegeHIS 125

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)

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)

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.

Loud noise results in damage to the cochlea OHCs are the 1st 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

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

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

Other Symptoms of NIHL Tinnitus Recruitment Distortion of sounds (this may result in

poorer than expected word recognition scores)

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

Factors that exacerbate NIHL: Simultaneous use of ototoxic drugs Chemical exposure

Certain solvents used in industrial applications Vibration