Conclusions and Comments
In this chapter the committee draws on the evidence it has reviewed to respond more directly to the specific points in its charge. The chapter also discusses potential opportunities identified by the committee to improve the effectiveness of the military services’ hearing conservation programs designed to prevent noise-induced hearing loss and tinnitus. Finally, research needs and opportunities suggested by the committee’s review are described. They cover the science of noise and hearing loss and tinnitus, as well as the protection of hearing and the course of hearing loss and tinnitus among military personnel.
RESPONDING TO THE ELEMENTS OF THE CHARGE
1. What sources of potentially damaging noise have been present in military settings since the beginning of World War II?
Many sources of potentially damaging noise have long existed in military settings. For the period addressed by this report—World War II to the present—some of these sources include weapons systems (e.g., hand guns, rifles, artillery pieces, rockets), wheeled and tracked vehicles, fixed- and rotary-wing aircraft, ships, and communications devices (Chapter 3). Service members may encounter these noise sources through training, standard military operations, and combat. Exposure to combat-related noise may be unpredictable in onset and duration. In addition, service members may be exposed to hazardous noise through activities that are not unique to the
military environment, including various engineering, industrial, construction, or maintenance tasks.
2. What levels of noise exposure are necessary to cause hearing loss or tinnitus?
The specific noise levels that cause noise-induced hearing loss vary with the duration of the exposure, the type of noise, and the frequency content of the noise, as well as the susceptibility of the exposed individual (Chapters 1 and 2). Time-weighted average noise exposures of approximately 85 dBA for 8 hours per day for a 40-hour work week, or the equivalent, are considered to be hazardous, but a person must be so exposed for a number of years before developing noise-induced hearing loss. On the other hand, impulse noise with peak levels exceeding approximately 140 dB SPL may be hazardous even for a single exposure. These guidelines for safe noise exposures are designed to protect the majority of individuals from noise-induced hearing loss, but not to ensure that every individual is protected. 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).
3. What is the evidence that hearing loss or tinnitus has been incurred by members of the armed services as a result of noise exposure during military service since World War II?
Patterns of hearing loss consistent with noise exposure can be seen in cross-sectional studies of military personnel (Chapter 3). Because large numbers of people have served in the military since World War II, the total number who experienced noise-induced hearing loss by the time their military service ended may be substantial, but the available data provide no basis for a valid estimate of the number. Neither was it possible to estimate the proportion of a given military population that developed noise-induced hearing loss or tinnitus during military service, the amount of hearing loss incurred, or the relative risk of noise-induced hearing loss or tinnitus for a given individual, based on his or her branch of military service, occupational specialty, or service era.
With regard to hearing loss, the majority of the data available are average group hearing thresholds from cross-sectional studies. These average data indicate that hearing thresholds are worse in those groups with more years of military service. However, these cross-sectional data are not a sufficient basis for attributing greater hearing loss solely to a longer exposure to noise while in the military. The timing of exposure to noise and the noise doses received (or other factors that may affect hearing loss) may
have differed among personnel who entered military service in different years by virtue of such factors as the timing of periods of combat or differences in use of hearing protection or recreational use of firearms.
Cross-sectional data can be used to identify associations between military service and noise-induced hearing loss, but are not sufficient to show causal relationships. In contrast, longitudinal data on hearing thresholds at the beginning and end of military service provide a basis for establishing that hearing loss occurred after exposure to noise during military service, a temporal ordering necessary for a causal relationship and estimation of risks.
With regard to tinnitus, even less information is available than for hearing loss (Chapter 4). The committee identified no epidemiological studies of tinnitus among U.S. military personnel, and the services’ hearing conservation programs do not include surveillance for tinnitus. Limited tinnitus surveillance was introduced in 2003 with post-deployment health assessments.
4. What is the evidence that the effects of noise exposure at younger ages can lead to delayed onset of noise-induced hearing loss later in life?
There is little evidence available with which to address this question (Chapter 2). No longitudinal studies have examined patterns of hearing loss over time in noise-exposed humans or laboratory animals who did not develop hearing loss at the time of the noise exposure. The committee’s understanding of the mechanisms and processes involved in the recovery from noise exposure suggests that a delay of many years in the onset of noise-induced hearing loss following an earlier noise exposure is extremely unlikely.
When hearing loss is known to have occurred as a result of a noise exposure, it has generally been thought that hearing loss for pure tones does not worsen following the cessation of a given noise exposure. However, there are no longitudinal data from humans who developed noise-induced hearing loss in early adulthood and were followed into their 60s, 70s, or 80s. Data from a few longitudinal studies of older adults, which differed in the way prior noise exposure was documented, have not produced conclusive results. To the committee’s knowledge, only one longitudinal study has examined changes in hearing in laboratory animals after a noise-induced hearing loss. In middle-aged gerbils that sustained a slight noise-induced hearing loss and were followed for most of their remaining lifetimes, no change in the amount of noise-induced hearing loss was seen over time.
It is possible, however, that an individual’s awareness of the effects of
noise on hearing may be delayed considerably after the noise exposure. As illustrated in Chapter 2, young adults with a slight noise-induced high-frequency hearing loss (e.g., 15–30 dB HL at 6000 Hz), one not likely to cause much difficulty with communication if present at the time a young adult might be discharged from military service, will likely exhibit greater hearing loss as they age than young adults with normal hearing (0 dB HL) at discharge. As demonstrated previously in Figures 2-6 and 2-7, a slight noise-induced hearing loss of 20–30 dB HL incurred as a young adult, when combined with a similar amount of hearing loss associated with aging alone, can become a moderate hearing loss of 40–50 dB HL at an age of 50 or 60 years. This amount of hearing loss is often sufficient to interfere with everyday communication, and it may make the individual more aware of the effects of the earlier noise-induced hearing loss, especially in comparison to same-aged peers without prior noise-induced hearing loss (who have approximately half as much hearing loss).
5. What additional risk factors for noise-induced hearing loss or tinnitus are supported by a good level of evidence?
In humans, no specific exogenous or endogenous factors were identified that correlated with increased susceptibility to noise-induced hearing loss or tinnitus (Chapters 2 and 4). Exogenous factors for hearing loss considered by the committee included exposure to aminoglycoside antibiotics, cisplatin, diuretics, salicylates, solvents, carbon disulfide, carbon monoxide, cigarette smoking, whole-body vibration, body temperature, exercise, and electromagnetic fields. Some of these agents (e.g., aminoglycoside antibiotics and cisplatin) are known to be ototoxins that may induce hearing loss unrelated to noise exposure. Studies in humans of the effect on hearing of exposure to any of these agents in combination with noise either have not been done or have not produced conclusive results. Endogenous factors considered by the committee included (old) age, gender, race, eye color, and prior hearing loss, but these factors did not correlate with increased susceptibility to noise-induced hearing loss.
The committee identified only one study in humans that had investigated the association between tinnitus and combined exposures to noise and other factors. Tinnitus risk factors, independent of noise exposure, include hearing loss, head injury, middle ear disease, and certain medications (e.g., salicylates, aminoglycoside antibiotics).
6. When were the military services’ hearing conservation measures adequate to protect the hearing of service members?
Data analyzed by the committee led to the conclusion that military
hearing conservation programs, dating from the late 1970s, cannot be considered adequate to protect the hearing of service members. The committee concluded that hearing conservation activities from World War II through the 1970s would have been even less adequate to protect the hearing of service members than programs in place since the 1980s, because only early hearing protection devices of limited effectiveness were available and mandatory hearing conservation measures were in place only in the Air Force (Chapter 5).
Given that engineering measures to reduce noise levels and administrative measures to reduce noise exposures may not be compatible with military operations, use of hearing protection devices is often the primary defense against noise-induced hearing loss for military personnel. The effectiveness of these devices has increased substantially since World War II, but still depends on how well and how often they are used. Data on the use of hearing protection by military personnel are limited, but a handful of reports over the past 30 years suggest that in some settings, only about half of those who should have been using hearing protection devices were doing so.
The services’ hearing conservation programs require annual measurements of hearing thresholds for military personnel who are considered to be exposed to hazardous noise. This surveillance effort alone will not prevent noise-induced hearing loss, but it may serve to limit the loss if the detection of temporary hearing losses or small permanent losses results in increased use of hearing protection or reassignment of the individual to lower noise environments. Available data show that records of hearing tests are being collected for only about half of the Army and Air Force personnel in the hearing conservation programs. Some personnel may not be receiving the required tests, and some test results may not be reaching the central hearing conservation registry system. The Navy and the Marine Corps do not report the proportion of enrollees who are being tested each year. The percentage of military service members tested each year who have a significant threshold shift (STS) has been approximately 10 percent in the Air Force over the past 3 years, and close to 18 percent in the Army, Navy, and Marine Corps, which is two to five times higher than rates considered appropriate in industrial hearing conservation programs.
7. When did the audiometric measures used by the military services become adequate to evaluate individual changes in hearing associated with military service?
A review of service medical records for veterans who left military service during the period from World War II to 2002 suggests that documented audiometric testing at entrance into and separation from service has
been and remains limited, even in the most recent eras (Chapter 6). As argued repeatedly in this report, it is critical to obtain an audiogram at entry into and exit from military service to clearly establish whether noise-induced hearing loss developed during military service. 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. As expected, the percentage of service medical records containing audiograms of any type was lowest for the period before 1950, except for the Air Force, an early leader in requiring the collection of audiograms. The results of the review of service medical records indicate that audiometric testing by the military services has not been adequate, throughout the period from World War II to the present, to evaluate changes in hearing associated with military service for the majority of service members.
OPERATIONAL NEEDS SUGGESTED BY THE REPORT
The current irreversibility of noise-induced hearing loss and tinnitus means that preventing these problems, or limiting their progression, is especially important. From the review of information on noise exposure in military settings, hearing loss and tinnitus experienced by some service members, and the hearing conservation activities of the military services, the committee identified several steps that may enhance hearing protection for service members and improve the effectiveness of the services’ hearing conservation programs. Although this report was prepared for the Department of Veterans Affairs (VA), it is the Department of Defense and the individual military services that can take these important steps to minimize the adverse effects of noise exposure on military personnel and better document hearing loss or tinnitus when either occurs during military service. The committee strongly recommends the following practices be implemented:
Work to achieve more extensive and consistent use of hearing protection by military personnel.
Include questions about the presence and severity of tinnitus in each ear on all audiometric records obtained from enlistment through the end of military service. (In the remaining suggestions, audiograms and audiometric records are assumed to include responses to questions about the presence and severity of tinnitus.)
Enforce requirements for audiograms prior to noise exposure for all new military service members at all basic training sites.
Enforce, and establish where they do not presently exist, require-
ments for audiograms at the completion of military service to ensure that any hearing loss or tinnitus arising during military service is adequately documented. The Department of Defense and the Department of Veterans Affairs should explore whether resources are available within the VA system to aid the military services in conducting audiometric tests and tinnitus assessments for personnel completing their military service.
Given the likely occurrence of maximum noise-induced hearing loss at 6000 Hz, include the measurement of hearing thresholds at 8000 Hz in all audiograms to allow for detection of the noise-notch pattern of hearing loss associated with noise exposure.
Enforce hearing conservation requirements for annual monitoring audiograms, as well as for follow-up audiograms if significant threshold shift is detected in annual monitoring audiograms.
Continue to develop the Defense Occupational and Environmental Health Readiness System (DOEHRS) to improve its reporting capabilities to match and exceed those available with the services’ previous systems. Further development of this system should include modification of the hearing conservation component (DOEHRS-HC) to track reports of tinnitus. It should also include implementation of the industrial hygiene component (DOEHRS-IH) to provide information on exposures to hazardous noise and other chemical, physical, biological, and ergonomic hazards.
Develop mechanisms to provide VA personnel access to records from DOEHRS-HC for review of disability claims for hearing loss or tinnitus that are not otherwise supported by audiometric records in the service medical record.
RESEARCH NEEDS SUGGESTED BY THE REPORT
The committee also saw areas where further research would be valuable for improving understanding of broad scientific questions concerning the relationship between noise exposure and hearing loss and tinnitus. Research could also address more targeted questions concerning noise exposure, hearing loss, tinnitus, and hearing conservation measures related to military service.
Two broad scientific areas were of interest to the committee:
Further investigate, both in laboratory animals and humans, exposures to fluctuating noise, impulse/impact noise, and combinations of noise, as well as intermittent exposures to steady-state noise, to determine the acoustic parameters associated with noise-induced hearing loss and tinnitus.
Further investigate the mechanism, natural history, epidemiology, measurement, and treatment of noise-induced hearing loss and tinnitus.
Several avenues of research specifically related to military settings and military personnel could be considered. Many are offered as a means to fill the void for prospective, longitudinal, epidemiological data on noise-induced hearing loss and tinnitus in military personnel.
Obtain valid estimates of the incidence, prevalence, and severity of noise-induced hearing loss and tinnitus among military personnel, including gender-specific estimates. If the reporting ability and completeness of existing databases, such as DOEHRS-HC, improve, greater use might be made of their data for analyses for personnel enrolled in hearing conservation programs.
Establish cohorts of military veterans with various documented noise exposures, immediately upon discharge, and survey them periodically for ototoxic exposures, subsequent nonmilitary noise exposures, and hearing function, as well as presence and severity of tinnitus, in order to determine whether there is a delay in the effects of military noise exposure. These cohorts will need to be followed through the remainder of members’ lifetimes, but this longitudinal study will reveal elements of the natural history of noise-induced hearing loss and tinnitus that otherwise will not be determined. The Millennium Cohort Study, which is designed to evaluate the long-term health of people who have served in the military, might provide a mechanism for conducting a longitudinal investigation of hearing health.
Conduct randomized trials of interventions within each military branch to determine with greater certainty which approaches to hearing conservation—including efforts to increase the use and effectiveness of hearing protection devices, compliance with requirements for audiometric testing, and the use of otoprotective medications—lead to lower incidence of noise-induced hearing loss and tinnitus.
On a sample basis, determine noise levels for modern military activities and also determine, with standard industrial hygiene methods, the noise dose experienced by individual military personnel where dosimetry has not been done.
Conduct real-world studies in military settings, including field and garrison conditions, to assess the noise attenuation and utilization rates of hearing protection devices, including the recently introduced earplugs that provide level-dependent sound attenuation.