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INTRODUCTION

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Occupational hearing loss may be partial or (rarely) total, unilateral or bilateral, and conductive, sensorineural, or mixed (conductive and sensorineural). Conductive hearing loss involves the external or middle ear, and impairs the passage of sound to the inner ear; sensorineural hearing loss (SNHL) results from dysfunction of the inner ear, auditory nerve, or brain. In the workplace, conductive and mixed hearing loss can be caused by blunt or penetrating head injuries, explosions, and thermal injuries such as slag burns sustained when a piece of welder's slag penetrates the eardrum. SNHL usually results from damage to the cochlea, especially loss of hair cells from the organ of Corti. Among the causes of occupational SNHL are continuous exposure to noise in excess of 85 dBA, blunt head injury, and exposure to ototoxic substances.

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PHYSIOLOGY OF HEARING

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Sound waves consist of alternating periods of compression and rarefaction within a medium such as air. The stronger the pressure variation, the louder the sound. Measurement of hearing in terms of sound pressure in micropascals (μPa) is cumbersome because of the enormous dynamic range of normal hearing (for the frequencies humans hear best, pressures between 20 and 20,000,000 μPa can be heard and tolerated). For this reason, the logarithmic decibel (dB) scale is used, compressing a million-fold pressure variation into a range of 120 dB. Since humans hear some frequencies better than others, audiometers are calibrated in “hearing level” (HL), a scale that defines 0 dB HL—at each frequency—as the faintest sound that the average healthy young person can detect.

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Sound frequency (the number of waves passing a fixed point each second, measured in Hertz [Hz]) correlates with pitch. The normal human ear can detect sounds across the frequency range from approximately 20–20,000 Hz. The most important range for human speech communication is between 500 and 3000 Hz.

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When sound traveling in air strikes water, almost all of it is reflected, because air and water have very different acoustic impedances. To allow efficient transmission of air-conducted sound into the fluid-filled inner ear, the impedance-matching middle ear has evolved; when the ear canal is open, and the tympanic membrane and the three ossicles (malleus, incus, and stapes) are working properly, the inner ear can respond to sounds that are up to 60 dB less intense than would otherwise be the case.

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The transduction of mechanical vibrations to nerve impulses by the inner hair cells takes place in the inner ear (cochlea) at the organ of Corti. The hair cells of the organ of Corti rest on the basilar membrane, and the stereocilia of the three rows of outer hair cells oscillate against the tectorial membrane. A shearing action between the stereocilia and the tectorial membrane, caused by the traveling wave motion of the basilar membrane, results in release of neurotransmitters by a single row of inner hair cells to the auditory nerve ...

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