Noise and Military Service Implications for Hearing Loss and Tinnitus (2006) / Chapter Skim
Currently Skimming:
1 Overview of the Problem and Introduction
1 Overview of the Problem and Introduction
Pages 15-32
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From page 15... ...
In addition, it is quite likely that an individual might have experienced other hazardous noise exposures subsequent to discharge from military service that could result in significant noise-induced hearing loss or tinnitus. After the fact, for example, there are no means currently available to distinguish the hearing loss resulting from several years of military service from the noise-induced hearing loss resulting from subsequent work in a noisy industry or from participation in a wide variety of recreational activities, such as hunting (e.g., Clark, 1991)
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From page 16... ...
Furthermore, this assessment frequently must be done in the absence of documentation of the measurement of hearing thresholds at or around the time of military service (see Chapter 6)
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From page 17... ...
on the assessment of noise-induced hearing loss and tinnitus associated with military service in the Armed Forces. The committee will review staff-generated data on compliance with regula tions regarding audiometric testing in the services at specific periods of time since World War II, review and assess available data on hearing loss in former service members, identify sources of potentially damaging noise during active duty, determine levels of noise exposure necessary to cause hearing loss or tinnitus, determine if the effects of noise exposure can be of delayed onset, identify risk factors for noise-induced hearing loss, and identify when hearing conservation measures were adequate to protect the hearing of service members.
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From page 18... ...
The reference sound pressure level (SPL) for computation of decibels related to acoustic measurements was selected so that 0 dB SPL corresponds approximately to the lowest mid-frequency sound pressure that can be heard by the average normal-hearing young adult under ideal free-field listening conditions.
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From page 19... ...
, rather than that associated with a single extreme noise exposure, which is more appropriately referred to as "acoustic trauma." Research over the past 60 to 70 years has shown that each of these acoustic parameters of noise -- its sound pressure level, duration, type (impulse versus steady-state) , and frequency content -- can influence the hearing loss that is measured following the exposure to noise.
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From page 20... ...
Although the noise dose remains constant regardless of the sequence of these three noise exposures, the resulting hearing loss varies significantly with the order of their presentation (Mills, 1992; Ward, 1991)
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From page 21... ...
As described in more detail in Chapter 2, a hallmark of noise-induced hearing loss is the appearance of a hearing loss for high-frequency sounds, with the worse hearing thresholds typically occurring at frequencies of 3000 6000 Hz. Frequently, hearing is normal or near normal at lower frequencies (< 1000 Hz)
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From page 22... ...
. Currently, damage to the ear as a result of noise exposure is not reversible in humans.
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From page 23... ...
Some key advantages of this approach to the study of the effects of noise on hearing include the use of human subjects rather than laboratory animals, precise control of the acoustical parameters of the noise exposure, and careful measurement of hearing thresholds under optimal listening conditions. The primary shortcoming of this approach has been in generalizing the results of short-term experiments on TTS to the permanent hearing loss resulting from years of repeated exposures or exposure to high noise levels.
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From page 24... ...
Specifically, noise exposures can be under strict control and both TTS and PTS can be measured in the same animals at various times during the animals' life spans. Furthermore, a variety of other measures beyond behavioral measurement of hearing thresholds, including a number of physiological and anatomical measurements, can also be obtained from the animals following completion of a
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From page 25... ...
APPROACHES TO HEARING CONSERVATION Three approaches can be taken to reduce the occurrence of noiseinduced hearing loss and tinnitus, whether from industrial or military exposures to noise. First, through engineering, the equipment or devices producing the noise can be redesigned to reduce the sound levels generated at the source.
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From page 26... ...
The participants in hearing conservation programs of the military are currently required to have hearing thresholds measured annually. Obviously, if this is not taking place, then an STS-based approach to hearing conservation will
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From page 27... ...
Another measure of program effectiveness is the incidence of STS among those individuals in the hearing conservation program. Ideally, STS-based approaches include steps to verify that observed STS values are not the result of TTS, usually through follow-up measures of hearing thresholds obtained after prescribed periods of quiet.
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From page 28... ...
Experimental studies of noise exposure and hearing loss or tinnitus in humans must be designed to prevent permanent harm. As a result, such studies can be conducted to study only those exposures resulting in temporary hearing loss or tinnitus.
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From page 29... ...
As noted previously, studies of noise-induced hearing loss in laboratory animals make it possible to examine the associations among TTS, PTS, and underlying cochlear damage in the same set of subjects and under strict laboratory control of the exposure. Such data represent a powerful tool for understanding the mechanisms underlying noise-induced hearing loss in a variety of mammalian species, including variables impacting the development of and recovery from noise-induced hearing loss, as well as establishing the relationships among TTS, PTS, and underlying pathology.
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From page 30... ...
However, when applying the foregoing scale for strength of evidence, the context of the specific question being addressed must be kept in mind. For example, if the specific question posed or the issue addressed pertains to the effect of noise on humans and the only evidence available is from studies of laboratory animals, this evidence is considered not to be sufficient regardless of the number of "strong" studies available from laboratory animals.
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From page 31... ...
1994. Oral magnesium intake reduces permanent hearing loss induced by noise exposure.
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From page 32... ...
1990. ISO 1999: Acoustics -- Determi nation of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Im pairment.
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