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June 7, 2020

Noise and Military Service: Implications for Hearing Loss and Tinnitus (2006); acoustic trauma; hearing loss or tinnitus incurred as a result of military service cannot be distinguished with certainty from subsequent noise-induced hearing loss or tinnitus resulting from work in a noisy industry or from participation in a variety of noisy recreational activities, such as hunting;

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Noise and Military Service: Implications for Hearing Loss and Tinnitus (2006)

Institute of Medicine 2006. Noise and Military Service: Implications for Hearing Loss and Tinnitus. Washington, DC: The National Academies Press. https://doi.org/10.17226/11443.

Table 3-1 provides examples of some of the measurements made since the 1950s of average sound levels found in ground vehicles and aircraft and peak sound pressure levels generated by certain weapons.3

The examples of noise levels associated with equipment and weaponry in the military included in Table 3-1 clearly demonstrate that there are many sources of high sound pressure levels in the military environment that exceed criteria for safe exposure. Data on sound pressure levels, however, are not sufficient by themselves to determine the noise dose received by an individual.

EXECUTIVE SUMMARY Pg 2 “After the fact, hearing loss or tinnitus incurred as a result of military service cannot be distinguished with certainty from subsequent noise-induced hearing loss or tinnitus resulting from work in a noisy industry or from participation in a variety of noisy recreational activities, such as hunting. Furthermore, high-frequency hearing losses are seen not only with noise exposure, but also at older ages (presbycusis), although the specific patterns of loss are generally distinguishable until 60–70 years of age (see Chapter 2). Tinnitus may also develop in response to factors other than noise exposure (e.g., head injury, brain tumors, middle ear diseases, certain medications) and can occur with or without hearing loss. If documentation of hearing thresholds or tinnitus during military service is not available, even a detailed case history from the veteran may leave considerable uncertainty about the association between a current hearing loss or tinnitus and prior military service.”

EXECUTIVE SUMMARY Pg 3 “… the phenomenon of acoustic trauma, which is the sudden loss of hearing following a single exposure to very hazardous noise.”

EXECUTIVE SUMMARY Pg 3 ” Noiseinduced hearing loss is often characterized by a “notch” in the audiogram, reflecting worse hearing at frequencies between 3000 and 6000 Hz than at lower and higher frequencies. The specific pattern of changes in pure-tone thresholds can vary depending on the type of noise exposure. The committee focused most of its attention on permanent changes in those thresholds. In adults, hearing loss is typically considered to be present when pure-tone thresholds are worse than 25 dB HL at any frequency usually tested.3”

EXECUTIVE SUMMARY Pg 3 “The study also included consideration of noise-induced tinnitus. Tinnitus is the perception of sound (e.g., ringing, buzzing, whistling) that cannot be attributed to an external sound source and is perceivable only by the person who is experiencing it. This subjective phenomenon is distinct from perceived sound that can be generated by events in the head or neck and that may be perceptible by an observer. The presence of tinnitus is determined primarily by self-report, but perceptual attributes, such as its pitch and loudness, can be established reliably under controlled conditions (psychoacoustic testing). The mechanisms underlying tinnitus are not fully understood. Some people with tinnitus experience serious problems associated with emotional well-being, sleep, hearing, and concentration. No current treatment will eliminate tinnitus, but some treatments may reduce its adverse impact.”

EXECUTIVE SUMMARY Pg 4 “Exposure to combat-related noise may be unpredictable in onset and duration.”

EXECUTIVE SUMMARY Pg 4 “On the other hand, impulse noise with peak levels exceeding approximately 140 dB SPL may be hazardous even for a single exposure. With regard to noise-induced tinnitus, specific parameters of hazardous noise exposure have not been defined, but noise levels associated with hearing loss are also likely to be associated with tinnitus (Chapter 4).”

EXECUTIVE SUMMARY Pg 6 “Tinnitus risk factors, independent of noise exposure, include hearing loss, head injury, middle ear disease, and certain medications (e.g., salicylates, aminoglycoside antibiotics).”

EXECUTIVE SUMMARY Pg 7-8 “The service medical records audited revealed that about 30 percent of personnel who left the Navy and Marine Corps during the period from the early 1980s to 2002 had both an entry and separation audiogram

within ±60 days of entry or separation, whereas the percentages were even lower, typically less than 12 percent, for personnel who had served in either the Army or the Air Force.

Chapter 3: Noise and Noise-Induced Hearing Loss in the Military

“The evidence is sufficient to conclude that, in the absence of audiograms obtained at the beginning and end of military service, it is difficult or impossible to determine with certainty how much of a specific individual’s hearing loss was acquired during military service.”

Chapter 4: Tinnitus

“The evidence is sufficient to conclude that hearing loss (hearing thresholds greater than 25 dB HL at one or more audiometric frequencies between 250 and 8000 Hz) is associated with a higher prevalence of tinnitus”

Overview of the Problem and Introduction

“If documentation of the existence of hearing loss or tinnitus at discharge from the military is missing, it is nearly impossible to determine whether hearing loss or tinnitus detected by audiometric testing later in life is the result of noise exposure during prior military service. Both noise and aging, for example, result in similar high-frequency hearing loss, although the specific patterns of hearing loss resulting from each are generally distinguishable until 60–70 years of age (see Chapter 2)”

NOISE AND MILITARY SERVICE,

pg 17-18 “It should be noted that Public Law 107-330 makes frequent reference to “acoustic trauma” in its charge to the committee (see Appendix A). At the committee’s initial meeting in May 2004, discussion with congressional staff members clarified that the intent of the legislation was not the study of “acoustic trauma,” which is a narrowly defined type of damage resulting from short-term, high-intensity noise exposure, but a study of the more broadly defined “noise-induced hearing loss,” of which acoustic trauma is a subtype.”

Pg 19-20 “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. The major influences of noise level and daily duration of exposure are captured in a single simplified metric, the noise dose. The noise dose represents the integration of noise level (more accurately, the underlying physical quantities) over the entire time of exposure. For a given exposure, the dose is of critical importance when evaluating the potential hazard to hearing of a particular noise. The primary importance of the noise dose was recognized many years ago by the scientific community and has been incorporated into national and international standards designed to estimate the noise-induced hearing loss resulting from noise exposure (ISO-1999 [ISO, 1990]; ANSI S3.44 [ANSI, 1996]). Most often, the noise dose is specified in terms of the 8-hour equivalent continuous noise level in dBA and is derived from the timeweighted average (TWA) of the underlying physical quantities (e.g., sound pressure). When establishing a specific noise dose, a device known as a noise dosimeter is used. Parameters built into the noise dosimeter that can impact the measured noise dose include a dosimeter-specific threshold level, below which sound levels will not be measured, a criterion level, and an exchange rate. The latter two parameters are prescribed by various noise standards. Currently, a criterion level of 85 dBA and an exchange rate of either 3 dB or 5 dB are among the most widely implemented values. The exchange rate describes the trading relation between sound level and exposure duration that yields equivalent hazard for successive halvings of the exposure duration. To illustrate the tradeoff between sound level and duration of exposure that is built into noise dosimetry, assuming a criterion level of 85 dBA and a 3-dB exchange rate, an 8-hour continuous exposure to steady-state noise at 85 dBA would have the same noise dose as 88 dBA for 4 hours, 91 dBA for 2 hours, or 94 dBA for 1 hour.”

Pg 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), the type of noise (steady-state, impulse/impact, blast), and the spectral content of the noise. Knowledge of values for each of these four parameters is necessary, but not sufficient, to fully assess the hazard of a given exposure to hearing.”

Pg 40-41 “Acoustic trauma can occur following exposure to very intense noise, typically blasts > 150 dBA. Humans experiencing blasts at very high sound levels (~ 180 dB SPL) may suffer damage to the middle ear, including hemorrhage in or perforation of the eardrum and fracture of the malleus (Davis et al., 1949; Hirsch, 1968; Ward, 1973; Henderson et al., 1974b; Roberto et al., 1989). If the eardrum does not rupture during such an intense exposure, the organ of Corti is likely to rupture off the basilar membrane (Ward, 1973; Henderson et al., 1974a,b; Roberto et al., 1989). When a portion of the organ of Corti ruptures, it does not reattach to the basilar membrane. 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.”

Pg 49 “An increased risk of hearing loss was also observed for aircraft maintenance personnel exposed to jet fuel and noise on an Air Force base (Kaufman et al., 2005).”

Pg 50 “Carbon monoxide is among the most common workplace air pollutants, especially for individuals working around gas-combustion engines, such as mechanics and other engine workers. Recent reviews of research in animal models indicate that low to moderate levels of carbon monoxide and hydrogen cyanide potentiate noise-induced hearing loss, especially at high noise levels (Cary et al., 1997; Fechter et al., 2000; also see Table D-2 in Appendix D). This potentiating effect, moreover, has been observed whether the exposures to each agent were simultaneous or successive. A recent review by Fechter (2004) notes that solvents (toluene, ethyl benzene, styrene) are likely to result in an additive effect to noise whereas asphyxiants (carbon monoxide, hydrogen cyanide) appear to result in synergistic effects to noise.”

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