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Noise and Military Service: Implications for Hearing Loss and Tinnitus 3 Noise and Noise-Induced Hearing Loss in the Military The focus of this chapter is on noise and noise-induced hearing loss in theU.S. military. The committee was asked to identify sources of potentially damaging noise in the military setting and to review and assess available evidence on hearing loss incurred by members of the armed services as a result of noise exposure during military service since World War II. Concern about noise exposure and hearing loss among military personnel has been evident throughout this period (e.g., Glorig, 1952; Carmichael, 1955; CHABA, 1968; Yarington, 1968; Walden et al., 1971; Yankaskas and Shaw, 1999). The first part of the chapter briefly reviews the services’ policies and programs to collect data on noise levels generated by equipment used by military personnel and the noise doses received by military personnel working in certain settings. Examples of the kinds of data collected through these efforts are provided. The remainder of the chapter focuses on the committee’s assessment of data on hearing thresholds and hearing loss among military service members since World War II. NOISE IN THE MILITARY ENVIRONMENT The sources of noise in the military are as varied as the activities carried out by the members of the Army, Navy, Air Force, Marine Corps, and Coast Guard. Obvious sources of potentially hazardous noise are weapons systems and jet engines, but vehicles, other aircraft, watercraft, communication systems, and industrial-type activities also serve as sources of potentially damaging noise.
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Collection of Data on Noise Levels and Estimated Noise Exposures Since World War II, numerous measurements of the sound pressure levels in proximity to various weapon systems and other military equipment have been collected. In addition, information has been collected on estimated noise doses for personnel working in steady-state noise. In the late 1970s, the Department of Defense (DoD) established, as part of an overall hearing conservation program, a department-wide requirement for periodic surveys of noise-hazardous environments and, subsequently, requirements for noise dosimetry. Each military service was responsible for collecting and maintaining information about hazardous noise environments and noise exposures. Many military sites had been collecting such information well before the DoD requirements were put in place. This section briefly reviews DoD-level requirements concerning measurement of noise levels and noise exposure. It also reviews the services’ data collection activities and the availability of these data. Noise-exposure limits are discussed in Chapter 5. Department of Defense Requirements In 1978, DoD established a requirement that each of the military services conduct sound surveys to identify and periodically monitor noise-hazardous environments (DoD, 1978). By 1987, the requirements included provisions for measuring noise exposures for workers exposed to noise levels of 85 dBA or more (DoD, 1987). Also included were separate specifications for the measurement of impulse noise and performance criteria for the measurement devices to be used. The current requirements, contained in DoD Instruction 6055.12, DoD Hearing Conservation Program, specify that sound pressure levels (SPLs) are to be measured in all potentially hazardous noise work areas at least once and within 30 days of any change in operations affecting noise levels (DoD, 2004). Noise exposure (i.e., dose) is to be measured as time-weighted average (TWA) noise levels for military personnel working in industrial-type operations with hazardous noise levels.1 The surveys must be conducted by trained personnel using sound-level meters or dosimeters meeting or exceeding relevant standards established by the American National Standards Institute (ANSI). The DoD instruction does not require measurement of noise doses associated with military activities, whether actual operations or training exercises. Noise exposure during such activities can be highly variable, and typical dosimeters are not designed to capture the rapid rise to briefly sustained peak sound pressure levels in excess of 140 dB that occur 1 The DoD instructions also apply to noise-exposed civilian workers.
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Noise and Military Service: Implications for Hearing Loss and Tinnitus with weapons fire and other impulse noise (e.g., Kardous and Willson, 2004). Requirements for maintaining data from noise surveys and exposure assessments have changed over time. The initial requirement in 1978 was that data be maintained for 5 years. By 1987, the period had been extended to 30 years. In 1996, DoD specified that noise exposure data were to be maintained “for the duration of employment plus 40 years” (DoD, 1996). DoD also has established design standards for noise levels of new materiel designed or purchased for the military services. In the most recent version of these standards (DoD, 1997), the stated purpose is to lead to equipment that minimizes noise-induced hearing loss, permits acceptable speech communication in a noisy environment, minimizes aural detection by an enemy, minimizes community annoyance, and provides acceptable habitability of personnel quarters (DoD, 1997). The design standards include limits for steady-state and impulse noise in occupied areas and noise from shipboard equipment and aircraft, including rotary-wing aircraft. However, for both new and older equipment used in “military-unique” settings,2 DoD regulations give priority to maintaining combat readiness and allow for tradeoffs between noise reduction and weight, speed, cost, or other factors crucial to the effectiveness of the equipment (DoD, 2004). Air Force The Air Force began requiring noise measurements in 1948 with its first regulation regarding hazardous noise: AFR 160-3, Precautionary Measures Against Noise Hazards (Gasaway, 1988). By 1956, regulations required the use of either direct measurement or published data to plot master plans of bases to indicate where exposure to hazardous noise might occur. Data on noise levels of aircraft and other power machinery were published periodically by the Wright Air Development Center at Wright-Patterson Air Force Base. One notable compendium contains measurements from within cockpit areas of hundreds of types and models of aircraft (Gasaway, 2002; also see Gasaway, 1986). Dosimetry measurements began at selected airbases in the late 1970s (Fairman and Johnson, 1979). A 1982 Air Force regulation required evaluation of individual noise exposures for personnel whose exposure exceeded 2 The term “military-unique” settings refers to DoD military and civilian operations and workplaces that are unique to the national defense mission (DoD, 1998). They include combat, combat training, and operation, testing, and maintenance of military equipment and systems, among which are weapons, aircraft, ships, submarines, missiles, ordnance, and tactical vehicles. The designation applies to such operations as peacekeeping missions, field maneuvers, naval operations, and military flight.
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Noise and Military Service: Implications for Hearing Loss and Tinnitus the equivalent of an 8-hour time-weighted average of 84 dBA (Department of the Air Force, 1982). For a group of employees doing similar work with similar noise exposures (referred to as a “similar exposure group”), it was and remains permissible to monitor the most highly exposed individual and to assign the resulting measurement to each group member, essentially a “worst case” assumption. Current noise evaluation procedures (Department of the Air Force, 1994) require dosimetry for a minimum of 3 worker-days (defined as one worker for 3 days, or three workers for 1 day) to identify the average daily exposure. Starting in the late 1980s, various Air Force installations began automating their recordkeeping for sound pressure levels and dosimetry. In 2005, each installation continues to maintain its own database. The dosimetry data are used primarily for local shop or worksite decisions, or occasional installation-level uses. Compilation of such data across the entire Air Force is possible but is not done for routine analysis (Weisman, 2005). Navy and Marine Corps Navy requirements for the collection of noise survey data date back at least to a 1983 regulation requiring noise measurements and personal dosimetry with appropriate equipment and calibration (Department of the Navy, 1983). The Navy is also responsible for noise surveys for Marine Corps facilities. To date, data on sound pressure levels are routinely collected at Navy and Marine Corps facilities but are not routinely transferred to a central database. Noise dosimetry data are routinely collected by local Navy medical units to perform exposure assessments and to make recommendations for placement of personnel into the hearing conservation program. Under current procedures, such noise exposure data, in the form of time-weighted average sound levels, must be provided to the exposed individuals, the command, and the entity providing medical surveillance (Navy Environmental Health Center, 2004b). Starting in 2002, noise dosimetry data collected for a variety of industrial, shipboard, and other naval operations, including Marine Corps activities, have been added annually to the Navy Occupational Exposure Database (Crowder, 2005). Some of these data date back to 1980. The data are not routinely used in the Navy’s overall hearing conservation program, but they are used on a case-by-case basis to respond to inquiries (Crowder, 2005). Army The Army has both a centralized program to evaluate the sound pressure levels of new weapons systems and equipment and a distributed pro-
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Noise and Military Service: Implications for Hearing Loss and Tinnitus gram for noise measurement and analysis at worksites at all Army installations. Through the centralized Health Hazard Assessment Program, begun in 1980, new equipment is tested to assess various potential hazards, including noise, chemicals, radiation, and vibration. The measurements of sound pressure levels are used to estimate likely time-weighted averages during use of the equipment, but noise dosimetry is not carried out as part of this program. The test information is used to make recommendations regarding the need for personal hearing protection as well as possible restrictions on training time with the systems (personal communication, F. Sachs, U.S. Army Center for Health Promotion and Preventive Medicine, August 18, 2004). The Army also has comprehensive data on sound pressure levels from weapons and equipment beginning from the 1970s and a more limited set of data going back to the 1960s. In addition, each Army installation evaluates work environments for potential noise hazards from steady-state noise in industrial-type operations. Since 1988, the sound pressure level and noise dosimetry measurements have been collected in the Health Hazard Information Module database of the Army’s Occupational Health Management Information System. Dosimetry measurements are not routinely attempted for military-unique activities in the Army, in part because the impulse noise components are not readily measured by current instrumentation (U.S. Army Center for Health Promotion and Preventive Medicine, 1999; personal communication, D. Ohlin, U.S. Army Center for Health Promotion and Preventive Medicine, 2005). Coast Guard Coast Guard noise surveys were part of the Coast Guard hearing conservation program by the late 1960s and early 1970s (McConnell, 2004). Sound pressure level and noise dosimetry measurements made by the Coast Guard are provided to units in the form of written reports (McConnell, 2005). Defense Occupational and Environmental Health Readiness System–Industrial Hygiene In 2005, as this report was being written, all the services were still using their own databases on sound pressure levels and noise dosimetry. However, development of a DoD-wide database for recording, storing, and retrieving sound pressure level and noise dosimetry data, as well as information related to other occupational exposures, is in advanced stages. Introduction of the Defense Occupational and Environmental Health Readiness System–Industrial Hygiene (DOEHRS-IH) is planned for fall 2005 (personal communication, K. Wisniewski, U.S. Army Center for Health
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Promotion and Preventive Medicine, February 2005). This database is intended to provide a longitudinal record of noise and other occupational exposures for DoD personnel. Sound Levels and Noise Doses in the Military Environment Information on noise sources and noise levels in the military environment is plentiful and detailed but not complete and not easily summarized. Sound levels vary depending on the distance from the sound source and the conditions under which the sound is being generated. Important characteristics of impulse noise include not only the peak sound pressure level, but the time pattern of the impulses and the frequency spectrum. 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 On aircraft carriers, flight operations create an environment with combinations of aircraft noise, mechanical noise, and impact noise (Yankaskas and Shaw, 1999). Below the flight deck, sound levels have been measured at 106 dBA during aircraft launches. Exposure to high sound levels has also been reported for military personnel in positions such as radio operators (Robertson et al., 1990) and sonar technicians (Marshall and Carpenter, 1988) in the Navy and cryptolinguists in the Air Force (Ritter and Perkins, 2001). In addition, military personnel may encounter potentially damaging noise from equipment and activities comparable to those found in industrial settings, such as the operation of heavy equipment (Chandler and Fletcher, 1983). Data are also available on the acoustic spectra of some types of noise in the military environment (e.g., Johnson and Nixon, 1974; Gasaway, 2002), but they are not illustrated in Table 3-1. 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. As described above, dosimetry data have also been collected, but the committee found little published dosimetry data that could be used to draw conclusions about typical exposures (e.g., Fairman and Johnson, 1979; Jordan and Jones, 1983). 3 Appendix F provides an illustrative list of documents, most of which are available in the published literature or in electronic form from government sources, that report sound levels generated by a variety of military aircraft, vehicles, equipment, and weapons systems.
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Noise and Military Service: Implications for Hearing Loss and Tinnitus TABLE 3-1 Examples of Sound Levels Associated with Military Equipment a. Aircraft in Use in the 1950s Name Model Location Condition Sound Level dBC/F Sound Level dBA Douglas Skyraider (USN; in use 1950s–1960s) A-1J (AD-7) Cockpit Takeoff 132 128 Climb 118 117 Level 121 120 Cruise 113 109 Douglas Skyraider (USN) A-1J (AD-7) Passenger area Taxi 87 81 Takeoff 111 111 Normal cruise 106 103 High cruise 109 108 NOTES: dBC/F, C-weighted or flat-weighted levels; dBA, A-weighted levels. SOURCE: Gasaway (2002). b. Military Equipment in Use in the 1960s Name Model Location Condition Sound Level dB Tanks Interior 115 (±10) Personnel carrier (APC) Interior 120 (±10) NATO rifle M-14 Operator’s right ear 20 rounds, full automatic 159 3.7-in rocket launcher (bazooka) Operator’s right ear 163 105mm howitzer 190 (impulse) Sergeant missile 100 ft from launch site 145 Helicopter (Shawnee) H-21C Crew chief, 15-20 ft from aircraft Rotor engaged 110 SOURCE: Adapted from Yarington (1968).
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Noise and Military Service: Implications for Hearing Loss and Tinnitus c.Flight Operations at Landing Signal Officer Platform, U.S. Navy Aircraft Carrier, 1970s Event (n) Aircraft Average Noise Level dB(SPL) Noise Level dBA Average Duration at ≥ 90 dBA (seconds per event) Trap (35) F4 128 122 5 Trap (1) A7 123 114 4 Trap (2) C1 114 108 5 Bolter (6) F4 120 116 5 Wave-off (8) F4 125 124 5 Touch and go (10) F4 129 121 4 Deck launch (2) C1 127 123 20 SOURCE: Robertson et al. (1978a). d. Military Equipment in Use in 2005 Army Vehicles Model Condition Location Speed km/hr (mph) Sound Level dBA High-mobility multipurpose wheeled vehicle (HMMWV), non-heavy variants M966, also: M996, M997, M998, M1037, and others 2/3 payload Crew positions 0(idle) 78 48(30) 84 88(55) 94 HMMWV, heavy variant M1097, M1097A2, M1113, M1114 2/3 payload Crew positions Up to 50(31) < 85 64(40) 88 80(50) 92 96(60) 98 HMMWV heavy variant M1097 Full payload Crew positions Up to 40(25) < 85 96(60) 100
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Army Vehicles Model Condition Location Speed km/hr (mph) Sound Level dBA Commercial utility cargo vehicle M1008, M1009, M1010, M1028 In cab Below 88(55) < 85 88(55) 85–91 Ambulance M1010 Patient areas All speeds < 85 Armored personnel carrier, A3 version M113A3 family, including M106A2, M1064A3, M1059A3, M58A3, M730A2, M901A3, M981A3 Idle 85–92 16(10) 106 32(20) 109 48(30) 114 63(40) 118 Abrams tank M1A2, M1, M1A1 In vehicle Idle 93 Tactical idle 103 16(10) 108 48(30) 114 63(40) 117 Army Helicopters Model Sound Level Location dBA Chinook CH-47D Cockpit 103 Blackhawk UH-60A Pilot 106 Copilot 106 Apache YAH-64 Pilot 104 Copilot 101
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Weapons (Impulse Noise) Model Condition Location Sound Level dBP 9mm pistol M9 Shooter 157 5.56mm rifle M16A2 Shooter 157 5.56mm squad automatic weapon fired from HMMWV M249 Gunner 160 7.62mm machine gun M60 Fired from HMMWV Gunner 155 0.50 caliber machine gun M2 Fired from HMMWV Gunner 153 Machine gun MK 19, Mod 3 Fired from HMMWV Gunner 145 Grenade M26 At 50 ft 164 MAAWS recoilless rifle M3 Gunner 190 Light antitank weapon M72A3 Gunner 182 Javelin antitank missile Open position Gunner 160 Enclosed position Gunner 166 Fighting position Gunner 172 105mm towed howitzer Ml 19 At charge 8 Gunner 183 155mm towed howitzer M198 Firing M203 propellant Gunner 178 NOTES: In flight, helicopter crews wear helmets with integral hearing protectors. Passengers must rely on their own hearing protectors (e.g., earplugs) or ones supplied by air operations. HMMWV, high-mobility multipurpose wheeled vehicle; MAAWS, multi-role anti-armor anti-personnel weapon system. SOURCE: Adapted from U.S. Army Center for Health Promotion and Preventive Medicine (2004d).
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Estimating Cumulative Noise Exposures Despite the existence of data on sound pressure levels generated by weapons and equipment, and dosimetry estimates of noise exposure for certain personnel, arriving at an estimate of the cumulative noise exposure of any service member or group of service members is nearly impossible. To an even greater extent than civilian workers, military personnel are not likely to experience homogeneous noise exposures over the course of their military service. The impulsive and intermittent nature of many military noise exposures (e.g., gunfire, plane launches and landings, tank operation, 6-month shipboard deployments) adds an element of uncertainty to considerations of exposure and effect. For example, “high noise levels” for about 3 hours of a 14-hour period have been described as typical of high-tempo flight operations on an aircraft carrier (Yankaskas and Shaw, 1999). As discussed in Chapter 2, intermittent noise exposure may permit recuperation, thus ameliorating to some extent the hazardous effect of noise exposures. However, military personnel may also have noise exposures that are prolonged compared to those of civilians. At sea, for example, sailors are exposed to ambient shipboard noise continuously and may encounter potentially hazardous noise levels even in their sleeping quarters, giving their auditory systems no opportunity for short-term recovery (Yankaskas and Shaw, 1999; Yankaskas, 2001, 2004). Even for personnel assigned to a specific occupational specialty, it is reasonable to assume that the typical activities for an individual vary over time and that the activities at any given time vary among personnel at different military installations. In addition, members of the military frequently change assignments and may be exposed to different degrees of noise hazard in different work settings. Among a sample of Navy enlisted personnel who served during the period 1982–2004 and were still serving after 2001, for example, the average length of service was 80 months, and time on shore duty averaged 40 months (Shaw and Trost, 2005). Finally, as described in detail in Chapter 5, wide variations in the effective use of hearing protection devices among military personnel can dramatically affect an individual’s noise exposure. In sum, despite the availability of data on sound pressure levels and some dosimetry data, the complexity of military noise exposures precludes ready estimates of service members’ cumulative noise doses. FINDING: The evidence is sufficient to conclude that hazardous noise levels are and have been present in many military settings. FINDING: Extensive collections of data on sound pressure levels produced by equipment and activities in military settings are available from World War II to the present. Many estimates of noise exposures (doses)
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Noise and Military Service: Implications for Hearing Loss and Tinnitus FIGURE 3-9 Percentages of each age group, from 18–24 years (left) to 55–64 years (right) having hearing thresholds greater than 25 dB HL at each of three pure-tone frequencies: 3000 Hz (3), 4000 Hz (4), and 6000 Hz (6). Dashed lines represent data from the better ear of Marines in the 1970s and the solid lines represent data for the better ear of men in an unscreened sample of the general population from the 1962 U.S. Public Health Service survey. SOURCES: Glorig and Roberts (1965); Goldenberg (1977). Another study, by Robertson and colleagues (1978b), measured hearing thresholds in 3,050 Navy military personnel and defined a significant high-frequency hearing loss as an average threshold at 3000, 4000, and 6000 Hz ≥ 30 dB HL. Among personnel with more than 5 years of service, 37 percent of those in the high-noise occupational specialties and 23 percent of those in the low-noise occupational specialties had a significant high-frequency hearing loss. Unfortunately, there are no comparable percentages from the 1962 USPHS sample to which these values from the Navy can be compared to determine if such percentages are greater than expected in the general population.
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Noise and Military Service: Implications for Hearing Loss and Tinnitus The Army and the Air Force include criteria for hearing in a classification system used to characterize the medical fitness of service members. For hearing, these ratings, known as profiles, vary from H-1 (closest to normal hearing) to H-4 (the most severe hearing loss). The Army’s current criteria for an H-1 profile are an average threshold in each ear of ≤ 25 dB HL for the frequencies 500, 1000, and 2000 Hz; no individual threshold > 30 dB HL at these frequencies; and a threshold of ≤ 45 dB HL at 4000 Hz (Department of the Army, 2003). Thus, having an H-1 profile means having normal or near-normal hearing for low and middle frequencies (500–2000 Hz) in both ears, but moderate hearing losses are possible at 4000 Hz and profound hearing losses are possible at 6000 and 8000 Hz. Thus, H-1 classification is not synonymous with “normal” hearing (see Chapter 1). Data for the Army from the 1970s for personnel in the infantry, armor, and artillery indicate that 20 to 30 percent were classified as H-2 or worse (Walden et al., 1971; Walden et al., 1975). Among a small group of recruits who had not begun basic training (n = 246), about 3 percent had H-2 hearing or worse. In a group of recruits who had completed their basic training (n = 255), 6 percent had H-2 hearing or worse. More recent data from the DOEHRS hearing conservation (DOEHRS-HC) database showed that from 1982 through 2003, only about 8 to 10 percent of Army personnel in the hearing conservation program were classified as H-2 or worse (U.S. Army Center for Health Promotion and Preventive Medicine, 2004b). The data suggest lower rates of hearing loss over the past 20 years compared to the mid-1970s, but the two populations are not necessarily comparable. For Air Force personnel in the hearing conservation program for the period 2000–2003, about 10 to 12 percent of those in the hearing conservation program were classified as H-2 hearing or worse (Air Force Hearing Conservation Registry, 2004a). Given that the H-1 classification permits moderate-to-profound high-frequency hearing loss, the percentages of personnel who do not meet the H-1 standard most likely underestimate the prevalence of hearing loss at high frequencies. Such losses are consistent with noise exposure, but from the H classification system alone, it is not possible to determine their etiology. In addition, these overall percentages do not account for any differences in the underlying age distributions or noise-exposure histories of the populations under consideration. Threshold Shifts The committee also examined reports on cases of STS and permanent threshold shift (PTS) provided by the hearing conservation programs of the Army, Navy, Air Force, and Marine Corps. Although the definition of STS used by the services has varied over time, the purpose of the measure is to
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Noise and Military Service: Implications for Hearing Loss and Tinnitus identify for follow-up individuals who demonstrate a clinically significant change in hearing thresholds relative to an earlier baseline (reference) measurement. An STS should be followed up with up to two additional measurements of hearing thresholds after a prescribed period of quiet. If the STS remains, or if the individual is not retested, the STS is classified as a PTS and a new reference is established to be used in subsequent assessments of STS. Reports of cases of STS are available for thousands of military personnel enrolled in the services’ hearing conservation programs, in some cases across several years. Data were available from the Army for 1983–2003 (U.S. Army Center for Health Promotion and Preventive Medicine, 2004c), from the Air Force for 1989–2004 (Air Force Hearing Conservation Registry, 2004c), and for the Navy and Marine Corps for 1999–2004 (Navy Environmental Health Center, 2004a). During this period, definitions of STS changed in 1987 and 1999. For example, for the period up to 1987, the Army defined STS as a change of at least 20 dB at 1000, 2000, 3000, or 4000 Hz or a change greater than 10 dB in the average hearing loss at 2000, 3000, and 4000 Hz in either ear. No age corrections were applied for the comparison with the reference thresholds. From 1987 to 1999, however, the Army used the same criteria, except that age corrections were applied. Applying age corrections would be expected to reduce the incidence of STS (and PTS), all else being equal, but a steady increase in reported STS occurred from 1987 through 1999. As of 1999, the Army (and the other branches of the military) eliminated age correction and the 20-dB individual-frequency criterion was reduced to 15 dB. The data from the Army show that roughly 6 to 8 percent of the audiograms obtained from military personnel in the hearing conservation program demonstrated positive STS each year from 1983 to 1987. The percentage of tested personnel who were found to have an STS has progressed steadily since 1987, and by 1999, the STS percentage had reached about 18 percent. It has remained at about that level since 1999. Since then, the STS percentage for the Navy and Marine Corps hearing conservation programs declined from levels of about 22–25 percent to levels of about 15–18 percent. In general, STS percentages have been lowest for the Air Force hearing conservation program, ranging from about 7 to 13 percent over the 15-year period for which data are available. Reports of PTS have generally followed similar patterns, with the Air Force again having the lowest percentages. Other STS data have also been reported. For example, the STS percentages for the Air Force hearing conservation program started at about 23 percent in 1975 and declined steadily to about 14 percent in 1979 (Department of the Air Force, 1980). However, a more complex definition of STS was employed in which the criterion amount of threshold change consid-
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Noise and Military Service: Implications for Hearing Loss and Tinnitus ered to be significant varied with the severity of hearing loss demonstrated in the reference thresholds. Wolgemuth et al. (1995) reported on STS incidence for a large sample (n = 12,492) of Navy personnel in the hearing conservation program of the Atlantic Fleet from 1987 to 1990. An overall STS incidence of 29 percent was reported using STS definitions equivalent to those used by the Army. This level is about three times greater than that for the Army from the same 1987–1990 period. It is not clear, however, whether age corrections were applied to the Navy data, as was the case for the Army. If not, this could account for some of the difference in STS incidence. In addition, the Army data are provided only for positive STS cases, and it is unclear if this was also true for the data from the Navy. A recent analysis conducted for the Navy (Shaw and Trost, 2005) used STS as an outcome variable to examine the effect of noise on hearing of Navy enlisted personnel (n ~ 251,000) during the period 1979–2004. STS was established based on the difference between a sailor’s earliest and latest hearing tests under the hearing conservation program. It was defined as an average change of 10 dB or more in thresholds at 2000, 3000, and 4000 Hz in either ear. In the absence of direct data on individual noise exposures, the analysis used data on occupational specialties and time spent assigned to various types of ships or other assignments to assess the combined effects on the risk of STS. The statistical analysis controlled for age, race, and gender. Overall, 11 percent of the study population was found to have an STS, which is about half the annual incidence values reported in DOEHRS-HC for the Navy’s hearing conservation program from 1999 to 2004. Shaw and Trost (2005) identified some variables that can influence the amount of STS observed. They reported that the time assigned to surface warships (e.g., aircraft carriers, battleships, destroyers, amphibious ships), for example, had a greater impact on STS than time assigned to other surface ships, certain submarine duties, or shore duties. Assignments to submarine engine rooms and Air Wings produced no greater risk than shore assignments, possibly because of ready access to and use of hearing protection. The probability of an STS reached 0.46 if the individual spent the entire study assigned to a surface war ship, a level significantly higher than the probability of 0.27 for individuals assigned entirely to shore duty. The foregoing review of STS data suggests that a substantial number of individuals may be experiencing declines of at least 10 dB in their hearing while in the military. It is not possible, however, to discern the causes underlying such changes (especially in the absence of age corrections), the time over which such shifts occurred (e.g., 1 year or 20 years), or the resulting degree of hearing loss associated with the shift (a shift from 0 to 15 dB HL, both levels reflecting normal hearing, or a shift from 20 to 35 dB
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Noise and Military Service: Implications for Hearing Loss and Tinnitus HL, from normal to mild hearing loss). Moreover, hearing thresholds at 6000 Hz are not considered in any definition of STS, yet the average data indicate that hearing loss among military personnel is likely to be greatest at this frequency. These data must be interpreted with considerable caution for other reasons as well. The DOEHRS-HC system is still experiencing difficulty in collecting data from each service. Percentages of individuals in the hearing conservation programs for whom reports of annual audiograms have been submitted to the DOEHRS-HC system have varied but have averaged only about 45 percent for the Army since 1998 and 55 percent for the Air Force since 2000 (Air Force Hearing Conservation Registry, 2004b; U.S. Army Center for Health Promotion and Preventive Medicine, 2004a; also see Chapter 5). Similar problems have been described for the Army data for the years preceding the implementation of the DOEHRS-HC system (Ohlin, 2004b). Moreover, the apparent incidence of PTS reflects not only threshold shifts that persist after retesting, but any cases in which the follow-up testing was not done, likely leading to an overestimate of the true incidence of PTS. EPIDEMIOLOGICAL STUDIES OF NOISE-INDUCED HEARING LOSS IN INDIVIDUALS WITH PRIOR MILITARY SERVICE The Vietnam Experience Study, conducted by the Centers for Disease Control (CDC, 1988a,b), included examination of hearing status among a random sample of male veterans who had enlisted in the U.S. Army during the period 1965 through 1971. The hearing levels of the men who had served in Vietnam (n = 2,490) were compared with those of men who had not served in Vietnam (n = 1,972). The mean age of the study participants in each group was 37 years. High-frequency hearing loss was defined as an average threshold at 3000, 4000, and 6000 Hz ≥ 51 dB HL. Individual cases were classified as positive or negative with regard to the presence of high-frequency hearing loss based on this definition. Six covariates were included in the logistic regression analysis that was performed on these data: age at enlistment, race, year of enlistment, enlistment status (volunteer versus draftee), score on a general technical test, primary military occupation, and preservice hearing loss. Of the veterans who served in Vietnam, 18 percent met the hearing-loss criterion for their left ears, compared with 13 percent of those who were not in Vietnam. Smaller percentages had qualifying hearing losses in their right ears or in both ears. Overall, the Vietnam veterans were 40 percent more likely to have high-frequency hearing loss in either ear alone or both ears than the veterans who had not served in Vietnam. Follow-up analyses indicated that this was driven primarily by those Vietnam veterans with
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Noise and Military Service: Implications for Hearing Loss and Tinnitus military tactical occupational specialties (e.g., infantry, artillery, armor), who were 2.5 times more likely than non-Vietnam veterans to have high-frequency hearing loss. Those veterans who served in Vietnam in nontactical occupational specialties were not at significantly greater risk for hearing loss than otherwise similar veterans who did not serve in Vietnam. These results illustrate the importance of exposure conditions, including combat, for identifying an elevated risk for hearing loss among Vietnam veterans. These data reflect hearing thresholds 15–20 years after service in Vietnam and include any effects of subsequent noise exposures or any selection effects that may have resulted in differences between the groups in subsequent noise exposure or survival. Noe et al. (2002) compared hearing thresholds among veterans and nonveterans using data from a community-based cohort study of older adults in Beaver Dam, Wisconsin. The average age for both groups was approximately 65 years. Hearing loss was defined as a pure-tone average hearing threshold of 25 dB HL or greater for 500, 1000, 2000, and 4000 Hz, and cases were then classified as being either positive or negative with regard to hearing loss. Covariates included in the subsequent logistic-regression analysis included age, longest held occupation, history of head injury, and smoking. Veterans (n = 999) were not found to be at greater risk for hearing loss than nonveterans (n = 588). In addition, mean hearing thresholds for the two groups were similar at all measured frequencies from 500 through 8000 Hz. This analysis does not attempt to address differences related to noise exposure, only differences associated with prior military service. Nonveterans, too, are subject to occupational noise exposure, and in this study prior military service as such is not associated with an increased risk of hearing loss in a population of older adults. FINDINGS In the more than 60 years since the U.S. entrance into World War II, over 25 million people have served in the U.S. armed forces. Their experiences, in five different services and at least five major conflicts, as well as peacetime eras, have exposed many to loud noise. These noise exposures are likely to have varied widely, even within similar occupational specialties and eras. Data and analyses to document and quantify noise exposures of military personnel during this period, as well as to document and quantify their hearing thresholds and permanent changes in those thresholds over the course of military service, are not available. The committee found only a limited number of studies on which to base its findings, and those studies were primarily for the period since 1970. Among these were cross-sectional studies showing patterns of hearing loss consistent with noise exposure, but no longitudinal studies that could pro-
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Noise and Military Service: Implications for Hearing Loss and Tinnitus vide reliable data on changes in individuals’ hearing thresholds over the course of military service. The available studies were not designed to be representative of a service as a whole and only rarely of a particular occupational group. Together, these factors made it impossible to generalize findings from these studies to broader populations of military service members or veterans or to personnel serving in other time periods. Furthermore, the variability of individual responses to noise exposure precludes using the average hearing thresholds reported for groups of study participants to estimate the hearing loss of individuals. FINDING: The evidence is sufficient to conclude that certain military personnel from World War II to the present have exhibited hearing thresholds while in the military that are typical of noise-induced hearing loss. FINDING: The evidence is not sufficient to reach conclusions regarding the number or proportion of service members, overall or in specific occupational groups or eras since World War II, who have experienced noise-induced hearing loss while in the military. FINDING: The evidence is not sufficient to determine the probability of acquiring noise-induced hearing loss associated with service in the military, or in specific branches of the military, for a given individual. The probability of acquiring noise-induced hearing loss can only be determined precisely with well-controlled, longitudinal epidemiological studies. FINDING: 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. REFERENCES Air Force Hearing Conservation Registry. 2004a. DOEHRS Data Repository: Air Force Hearing Profile by Pay Grade, 2000–2004. Data provided to the Institute of Medicine Committee on Noise-Induced Hearing Loss and Tinnitus Associated with Military Service from World War II to the Present, Washington, DC. Air Force Hearing Conservation Registry. 2004b. DOEHRS Data Repository: Hearing Conservation Compliance Report, 2000–2003. Data provided to the Institute of Medicine Committee on Noise-Induced Hearing Loss and Tinnitus Associated with Military Service from World War II to the Present, Washington, DC. Air Force Hearing Conservation Registry. 2004c. DOEHRS Data Repository: Positive Significant Threshold Shift, 1999–2004. Data provided to the Institute of Medicine Committee on Noise-Induced Hearing Loss and Tinnitus Associated with Military Service from World War II to the Present, Washington, DC.
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Representative terms from entire chapter: