Noise and Military Service Implications for Hearing Loss and Tinnitus (2006) / Chapter Skim
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2 Noise-Induced Hearing Loss
2 Noise-Induced Hearing Loss
Pages 33-71
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From page 33... ...
The effects of noise on hearing thresholds are reviewed next, followed by a review of the time course for the development of hearing loss from noise exposure. Next, exogenous and endogenous risk factors that may alter an individual's susceptibility to noise-induced hearing loss are reviewed.
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From page 34... ...
The primary sensory receptors for hearing, the inner hair cells, are found within the organ of Corti as are the outer hair cells, which primarily facilitate the sensory response of the inner hair cells. The pressure waves within the cochlea vibrate the basilar membrane and the attached organ of Corti (Figure 2-2)
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From page 35... ...
The mechanical activity of the basilar membrane leads to mechanical stimulation of the inner and outer hair cells. From the surface of each hair cell, thin hair-like processes (stereocilia)
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From page 36... ...
Effects of Noise on Hearing The magnitude of hearing loss that results from excessive exposure to noise depends on factors associated with the exposure (e.g., sound pressure level [SPL] , duration, type of noise, and frequency)
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. However, above these peak sound pressure levels, the auditory system is damaged primarily by the large displacements caused by high peak levels.
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Intermittent and Continuous Exposures to Steady-State Noise Exposure to less intense noise (i.e., < 90 dBA) for short durations (i.e., 24 hrs)
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From page 39... ...
, a pattern that typifies a noise notch, this is not always readily apparent for individual data. Discerning a noise notch in the pattern of hearing loss may be especially challenging in older adults for whom age-related hearing loss is superimposed on a preexisting noise notch (see pp.
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From page 40... ...
In summary, there are four key acoustic parameters of a given noise exposure that determine the type and amount of the resulting hearing loss. These are the sound pressure level of the noise, the duration and temporal pattern of the exposure (hours/day, impulses/day, number of years)
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From page 41... ...
Rather, it eventually degenerates. As noted, the hearing loss associated with acoustic trauma often is severe and spans a wide range of frequencies, much broader than that represented by the high-frequency, noise-notch pattern of hearing loss associated with other types of noise exposures.
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From page 42... ...
42 ise outer no amaged and noise moderate (IHC) undifferentiated, (C)
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From page 43... ...
. The pattern of hearing loss measured following noise exposure provides valuable information about the extent and severity of the underlying damage, especially in the middle and high frequencies following exposures to broad-band sounds.
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From page 44... ...
Thus, conclusions about cochlear damage resulting from noise exposure in humans are limited by the imperfect association between behavioral measures of hearing threshold and underlying cochlear damage noted previously (e.g., Bredberg, 1968)
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From page 45... ...
That is, thresholds always improve or remain stable as postexposure time increases. Limited data on PTS in humans appear to be consistent with these data from laboratory animals on recovery following noise exposure.
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From page 46... ...
, on the other hand, had increased by about 10 20 dB over this same time period. In other words, at an age of 36 months, hearing thresholds in the ears subject to both noise exposure and aging were nearly equivalent to the hearing levels in the control ears subject only to aging (and substantially lower than the thresholds predicted using ISO/ ANSI rules for the combined effects of noise and aging; see below)
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From page 47... ...
Most recovery to stable hearing thresholds occurs within 30 days. FINDING: There is not sufficient evidence from longitudinal studies in laboratory animals or humans to determine whether permanent noiseinduced hearing loss can develop much later in one's lifetime, long after the cessation of that noise exposure.
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From page 48... ...
Finally, the studies reviewed included a wide range of methods to assess hearing loss. With these caveats in mind, we proceed to a review of evidence on the effects of certain exogenous and endogenous factors on the amount of noise-induced hearing loss measured following noise exposure.
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From page 49... ...
Nonoccupational noise exposures are also difficult to estimate and control for in analyses. In addition, workers of different ages or with different durations of employment
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From page 50... ...
Exposure to chemical asphyxiants, such as hydrogen cyanide and carbon monoxide, is common in some workplaces, and the interaction of these asphyxiants with noise exposure has also received attention recently. Cyanides are used in electroplating and metal leaching.
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From page 51... ...
In particular, the evidence is not conclusive in humans with regard to additive or synergistic effects of noise and the following exogenous factors on hearing: aminoglycosides, cisplatin, diuretics, salicylates, solvents, carbon disulfide, carbon monoxide, cigarette smoking, whole-body vibration, body temperature, exercise, and electromagnetic fields. Endogenous Factors Previous reviews of endogenous factors affecting noise-induced hearing loss have been performed (Humes, 1984; Henderson et al., 1993; Ward, 1995)
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From page 52... ...
Army and noted that, for similar noise exposures and length of service exceeding 10 years, black soldiers demonstrated less age-corrected noiseinduced hearing loss than white soldiers. It is unclear, however, whether this is a valid indication of racial differences in noise susceptibility.
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From page 53... ...
An individual who enters this noise exposure with a hearing threshold of 0 dB HL at 4000 Hz will demonstrate a threshold shift of 20 dB, whereas another individual who enters this noise exposure with a hearing threshold of 40 dB HL at 4000 Hz will show a threshold shift of 0 dB. On the other hand, if a given noise exposure is known to produce a noise-induced hearing loss of 40 dB HL at 4000 Hz, individuals with preexposure thresholds of 0 and 20 dB HL at 4000 Hz will demonstrate, on average, threshold shifts of 40 and 20 dB, respectively.
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From page 54... ...
Basically, the nonlinear additivity rules used to combine hearing thresholds from age-related hearing loss and noise-induced hearing loss, reviewed in detail below, also apply to the addition of previous and current noise-induced hearing loss. The basic conclusion from review of the research in this area is that individuals with previous noise-induced hearing loss are neither more nor less susceptible to subsequent noise-induced hearing loss than individuals without such pre-existing hearing loss.
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From page 55... ...
, noise exposures (e.g., daily exposures that differ significantly from 8 hours) , or exposure durations (less than 10 or more than 40 years)
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From page 56... ...
. The 8-hour equivalent continuous noise level is the parameter in the top panel, whereas the noise exposure is fixed at the highest level (100 dBA)
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From page 57... ...
Nonetheless, the noise-notch pattern of hearing loss is maintained and the average hearing loss associated with noise exposure only is about 40 dB HL in the high frequencies after 40 years of exposure to the highest noise level included in the ISO/ ANSI standards. One of the key issues covered in the ISO/ANSI standards is the way each addresses the combined effects of noise-induced and age-related hearing loss.
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From page 58... ...
The model used in the standards also is similar to a more general nonlinear compressive model that has proven successful when adding or combining elevated thresholds of various types (Humes et al., 1987; Humes and Jesteadt, 1989) , including those arising from noise exposures (Humes and Jesteadt, 1991)
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From page 59... ...
This serves to underscore the critical importance of periodic measurement of hearing thresholds for those exposed to high levels of noise. Given the wide range of individual differences in susceptibility, the possible effects of exogenous factors, and our inability to predict these effects from other measures, regular measurement of hearing thresholds is the only way to determine if a change in hearing has occurred in an individual during the period of a particular noise exposure.
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From page 60... ...
The ISO and ANSI standards both make provision for the use of other databases representing age-related hearing loss. Database B in both standards represents the hearing thresholds for a large unscreened sample from a U.S.
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From page 61... ...
. The bottom panel displays hypothetical NIPTS in dB, as well as the noise-only hearing loss data plotted in dB HL.
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From page 62... ...
To examine the impact of aging on the pattern of hearing loss across frequency, the additivity model of ISO-1999 and ANSI S3.44 was applied to the two sets of data in the top and bottom panels of Figure 2-6. The top panel was assumed to represent a "pure" age-related hearing loss for each age decade, and the bottom panel was interpreted as four different degrees of noise notch developed in four young men during the first few years of noise exposure (rather than the progression of noise-induced hearing loss over time, as originally indicated)
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From page 63... ...
With only the hearing thresholds from a much later stage in life, it is virtually impossible to discern how much, if any, of an individual's hearing loss can be attributed to noise exposure or for how long this hearing loss might have been present.
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From page 64... ...
The combined effects of noise and age included in ISO-1999 and ANSI S3.44 lie somewhere between these two extremes. FINDING: The evidence from cross-sectional studies of noiseinduced hearing loss in humans is sufficient to conclude that daily timeweighted average noise exposures greater than approximately 85 dBA for 8 hours for periods of many years pose a hazard to human hearing and that the hazard increases as the time-weighted average exposure exceeds this value.
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From page 65... ...
1972. Temporary threshold shifts from prolonged exposure to noise.
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From page 66... ...
1964. Permanent threshold shift changes produced by noise exposure and aging.
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From page 67... ...
1990. ISO 1999: Acoustics -- Determi nation of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Im pairment.
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1994. The application of frequency and time domain kurtosis to the assessment of hazardous noise exposures.
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From page 69... ...
1998. Assessing occupational hearing loss: Beyond noise exposures.
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1982. Species difference in susceptibility to noise exposure.
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From page 71... ...
2003. The role of music intensity in aerobics: Implications for hearing conservation.
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