Noise in Industry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 616

prominent member; exposure to certain chemicals and industrial solvents; hereditary factors; head
trauma; sudden hyperbaric- or altitude-induced pressure changes; and aging of the ear (presbycusis).
Furthermore, not all noise exposure occurs on the job. Many workers are exposed to hazardous levels
during leisure activities, from such sources as automobile

motorcycle racing, personal stereo headsets
and car stereos, firearms, and power tools. The effects of noise on hearing are generally subdivided
into the following three categories. 15

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31.3.3.2.1 Acoustic Trauma
Immediate organic damage to the ear from an extremely intense acoustic event such as an explosion is
known as acoustic trauma. The victim will notice the loss immediately and it often constitutes a perma-
nent injury. The damage may be to the conductive chain of the ear, including rupture of the eardrum or
dislodging of the ossicles (small bones) of the middle ear. Conductive losses can, in many cases, be com-
pensated for with a hearing aid or surgically corrected. Neural damage may also occur, involving a dis-
lodging of the hair cells or breakdown of the neural organ (Organ of Corti) itself. Unfortunately, neural
loss is irrecoverable and not typically compensable with a hearing aid. Acoustic trauma represents a
severe injury, but fortunately its occurrence is uncommon, including in the industrial setting.

31.3.3.2.2 Noise-Induced Threshold Shift
A threshold shift is defined as an elevation of hearing level from the individual's baseline hearing level and
it constitutes a loss of hearing sensitivity. Noise-induced temporary threshold shift (NITTS), sometimes
referred to as “auditory fatigue,” is by definition recoverable with time away from the noise. The elevation
of threshold is temporary, and usually can be traced to an overstimulation of the neural hair cells (actu-
ally, the stereocilia) in the Organ of Corti. Although the individual may not notice the temporary loss of
sensitivity, NITTS is a cardinal sign of overexposure to noise. It may occur over the course of a full
workday in noise or even after a few minutes of exposure to very intense noise. Although the relation-
ships are somewhat complex and individual differences are rather large, NITTS does depend on the level,
duration, and spectrum of the noise, as well as the audiometric test frequency in question. 15
Prevention of noise-induced permanent threshold shift (NIPTS), for which there is no possibility of
recovery, is the primary target of the industrial hearing conservationist. NIPTS can manifest suddenly
as a result of acoustic trauma; however, industrial noise problems that cause NIPTS most typically con-
stitute exposures that are repeated over a long period of time and have a cumulative effect on hearing
sensitivity. In fact, the losses are often quite insidious in that they occur in small steps over a number
of years of overexposure and the worker is not aware of the problem until it is too late. This type of
exposure produces permanent neural damage, and although there are some individual differences as
to the magnitude of loss and audiometric frequencies affected, the typical pattern for NIPTS is a promi-
nent elevation of threshold at the 4000 Hz audiometric frequency (sometimes called the 4-kHz notch),
followed by a spreading of loss to adjacent frequencies of 3000 and 6000 Hz. From a classic study on
workers in the jute weaver industry, Figure 31.3 depicts the temporal profile of NIPTS as the family
of audiometric threshold shift curves, with each curve representing a different number of years of
exposure. 16 As noise exposure continues over time, the hearing loss will spread over a wider frequency
bandwidth inclusive of midrange and high frequencies, and encompassing the range of most auditory
warning signals. In some cases, the hearing loss renders it unsafe or unproductive for the victim to
work in certain occupational settings where the hearing of certain signals are requisite to the job. Unfor-
tunately, the power of the consonants of speech sounds, which heavily influence the intelligibility of
human speech, also lie in the frequency range that is typically affected by NIPTS, compromising the
victim's ability to understanding speech. This is the tragedy of NIPTS in that the worker's ability to com-
municate is hampered, often severely and always irrecoverably. Furthermore, unlike blindness or many
physical disabilities, hearing loss is not overt and therefore often goes unrecognized by others. Thus, it is a
particularly isolating disability because the victim is unintentionally excluded from conversations and
may miss important auditory signals because others either are unaware of the loss or simply forget
about the need to compensate for it.