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Year : 2011  |  Volume : 13  |  Issue : 50  |  Page : 76--83

Analysis of army-wide hearing conservation database for hearing profiles related to crew-served and individual weapon systems

William A Ahroon1, Melinda E Hill1, Dennis P Goodes2,  
1 US Army Aeromedical Research Laboratory, 6901 Farrel Road, Fort Rucker, AL 36362-0577, USA
2 General Dynamics Information Technology, 504 Scott Street ATTN: MCMR-RTO, Fort Detrick, Maryland 21702-5012, USA

Correspondence Address:
William A Ahroon
US Army Aeromedical Research Laboratory, 6901 Farrel Road, Fort Rucker, AL 36362-0577


Damage-risk criteria (DRC) for noise exposures are designed to protect 95% of the exposed populations from hearing injuries caused by those noise exposures. The current DRC used by the US military follows OSHA guidelines for continuous noise. The current military DRC for impulse exposures follows the recommendations from the National Academy of Sciences - National Research Council Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) and are contained in the current military standard, MIL-STD-1474D DQNoise Limits.DQ Suggesting that the MIL-STD for impulse exposure is too stringent, various individuals have proposed that the DRC for exposure to high-level impulses be relaxed. The purpose of this study is to evaluate the current hearing status of US Army Soldiers, some of whom can be, by their military occupational specialties (MOS), reasonably expected to be routinely exposed to high-level impulses from weapon systems. The Defense Occupational and Environmental Health Readiness System - Hearing Conservation (DOEHRS-HC) was queried for the hearing status of enlisted Soldiers of 32 different MOSs. The results indicated that less than 95% of the Soldiers in the DOEHRS-HC database were classified as having normal hearing. In other words, the goal of the DRC used for limiting noise injuries (from continuous and impulse exposures) was not stringent enough to prevent hearing injuries in all but the most susceptible Soldiers. These results suggest that the current military noise DRC should not be relaxed.

How to cite this article:
Ahroon WA, Hill ME, Goodes DP. Analysis of army-wide hearing conservation database for hearing profiles related to crew-served and individual weapon systems.Noise Health 2011;13:76-83

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Ahroon WA, Hill ME, Goodes DP. Analysis of army-wide hearing conservation database for hearing profiles related to crew-served and individual weapon systems. Noise Health [serial online] 2011 [cited 2020 Jul 9 ];13:76-83
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Health hazard assessments are intended to evaluate the risk of injury to individuals from materiel employed during the performance of their duties. The rules employed to conduct these assessments are frequently called damage risk criteria (DRC). A DRC specifies the likelihood of a specific type of injury and a limit not-to-exceed to prevent the injury. For example, the Occupational Health and Safety Act (OHSA) rule for continuous noise exposure specifies that an individual may not be exposed to more than a daily eight-hour exposure to noise at more than 85 A-weighted decibels (dBA). Furthermore, as the exposure intensity increases by three decibels (dB), a doubling of exposure energy, the maximum permissible exposure duration is halved.

As suggested above, DRCs include two components - the injuries which the criteria are intended to prevent and the risk of occurrence of these injuries. The determination of the risk of hearing loss from noise exposure acknowledges that some individuals are more susceptible to injury from noise than others. The assignment of a number or proportion to the risk is based on subject matter experts' opinions regarding the size of the susceptible population and the importance of protecting the hearing of the largest percentage of individuals. For example, the 1965 US Army Human Engineering Laboratory's DRC [1] was intended to protect 75% of exposed individuals. Likewise, Rice and Coles [2] DRC for impulsive sounds protected 90% of the population. Coles et al.'s [3] DRC was designed to protect 75% of the population but the authors noted that 90% could be protected if the impulse peak levels were reduced by 10 dB. The National Academy of Science-National Research Council Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) DRC, [4] from which the MIL-STD-1474D [5] Requirement 4 was derived, was designed to protect "all but the most susceptible five percent of exposed individuals" (p. 2), thus protecting 95% of the population (for normal-incidence impulses). In recent considerations of MIL-STD-1474D Requirement 4, [6],[7],[8] the risk selected has followed the CHABA protection risk, that is, to protect 95% of the exposed population.

During the development of DRCs for noise exposure, it is generally assumed that a noise exposure that causes a certain amount of temporary threshold shift (TTS) following a single exposure will result in that amount of permanent hearing loss (permanent threshold shift, PTS) if the exposure is allowed to continue for 10 or more years. The acceptable amounts of TTS (acceptable for 95% of the population) for the CHABA [4] DRC was 10 dB at 1 kHz), 15 dB at 2 kHz, and 20 dB at 3 kHz or above, resulting in permanent losses of no more than 10, 15, and 20 dB at 1, 2, and 3 kHz over a working lifetime.

Among the US military services, there is no clearly defined damage risk criterion for impulsive noise. Many regulations note that impulses equal to or greater than 140 dB peak sound pressure level (SPL) are hazardous and military personnel are not to be exposed to impulses above this level unless hearing protection is worn. [9],[10],[11],[12]

The Department of Defense Design Criteria Standard MIL-STD-1474D [5] is a materiel design standard that defines the noise limits for military materiel and specifies the various avenues to measure, analyze, and report noise levels. It contains requirements for steady-state noise, aural non-detectability, community annoyance, impulse noise, shipboard equipment noise, and fixed- and rotary-wing aircraft noise. Requirement 4, "Impulse Noise," defines the standard for personnel exposed to impulses. Its scope, applicability, and purpose are

"1.1 Scope. This Requirement establishes impulse noise limits and prescribes testing requirements and measurement techniques for determining conformance to impulse noise limits.

1.2 Applicability. This Requirement applies to the acquisition and product improvement of all designed, or purchased (non-developmental items) systems, subsystems, equipment, and facilities that emit impulse noise. This standard is intended to address impulse noise levels emitted during the full range of typical operational conditions.

1.3 Purpose. This Requirement provides criteria for designing and selecting materiel having impulse noise levels that minimize noise induced hearing loss" (p. 35).

Even though the MIL-STD-1474D is a design criteria standard that defines damage ("minimize noise-induced hearing loss") and does not address risk, Requirement 4 serves as the de facto damage risk criterion for impulse noise for the military. It defines the maximum number of rounds to which military personnel may be exposed per day depending on the peak level of the impulse, its B duration, and the hearing protection used. (The B duration is the total time, typically measured in milliseconds [ms], that the envelope of the pressure-time waveform of the impulse is within 20 dB of the peak pressure level.) All hearing protectors are treated the same by MIL-STD-1474D. [Figure 1] displays the MIL-STD-1474D peak SPLs and B durations for impulses.{Figure 1}

The number of daily impulses (e.g. rounds) to which military personnel may be exposed for various peak pressures, B durations, and hearing protection are presented in [Table 1]. For example, for an impulse of 170 dB peak SPL and B duration of 30 ms (i.e. between the X and Y curves), 100 rounds per day are permitted if single hearing protection is used and 2000 rounds per day are permitted if double hearing protection is used. Unprotected personnel may not be exposed to this 170 dB peak SPL impulse. No exposures above the Z line in [Figure 1] are permitted, protected or unprotected.{Table 1}

Other methods of estimating hazard from exposure to high-level impulses have been proposed by Dancer et al., [13] Chan et al., [6] Price [8] and others. Furthermore, a number of studies have suggested that MIL-STD-1474D is too stringent and should be relaxed. [6],[14],[15],[16],[17],[18],[19] As a result of the US Army Medical Research and Materiel Command (MRMC) sponsored blast overpressure studies reported by Johnson [19] (the "Albuquerque Study"), a free-field exception to MIL-STD-1474D was promulgated by the US Army Center for Health Promotion and Preventive Medicine (CHPPM) permitting higher peak-level exposures when the following conditions were met.

The weapon is to be fired in the open.The pressure-time history contains at most two significant peaks, an incident pressure peak which may be followed by a ground-reflected peak. Only peaks within 50% of the highest peak will be considered significant.The A-duration is between 2 and 6 ms.The B-duration is less than 60 ms.

While some of the proposed changes or replacements for Requirement 4 are ambiguous with regard to increasing or relaxing the permitted daily exposures, some proponents of alternate methods argue that the MIL-STD-1474D Requirement 4 be relaxed and suggest that higher levels of impulses actually provide no greater risk of hearing loss than the existing military standard permits. [6,8] Before the DRC can be relaxed, however, it is prudent to evaluate the existing population of exposed individuals to determine how effective the current DRC is at controlling noise-induced hearing loss (NIHL). There have been no calls for the DRC for continuous noise to be relaxed or modified. Rather, discussions regarding continuous noise hazards focus on the appropriate selection, fitting and use of hearing protection devices, and the synergistic effects of other factors (e.g. inhalants, vibration, etc.).

Hearing acuity is captured on annual hearing screenings and stored within the Defense Occupational and Environmental Health Readiness System-Hearing Conservation (DOEHRS-HC) and can be recorded on form DD 2215 and DD 2216 in a Soldier's medical record. This information encompasses low to high frequencies from 500 to 6000 Hz, the frequencies most important for speech intelligibility. [20],[21] Hearing readiness is described in the US Army Standards of Medical Fitness, Army Regulation [AR] 40-501 [22] as a Soldier's "hearing profile." H-1 and H-2 hearing profiles are defined in terms of audiometric threshold only: H-1-average threshold at 500, 1000, and 2000 Hz not more than 25 dB HL, with no individual level greater than 30 dB HL at these frequencies, and threshold at 4000 Hz not more than 45 dB HL; H-2-average threshold at 500, 1000, and 2000 Hz not more than 30 dB HL, with none of these individual levels greater than 35 dB HL and threshold at 4000 Hz not more than 55 dB HL. The poorer ear may be deaf in an individual assigned with an H-2 hearing profile.

If a Soldier's results fall into an H-1 profile, the Soldier is considered to have acceptable hearing sensitivity. H-2 profiles suggest a mild-to-moderate hearing loss for at least the better ear. These individuals are likely to experience difficulty in hearing speech clearly in some quiet and most noisy listening situations. Once audiometric thresholds exceed those specified for an H-2 hearing profile, the Soldier would be classified with at least an H-3 hearing profile. H-3 hearing profiles have many different audiometric configurations but the final determination rests on the performance of the individual in a speech reception threshold test (SRT). The SRT is a test performed by an audiologist in a sound-treated booth utilizing spondees (two-syllable words with equal emphasis on both syllables) that the Soldier must repeat at consecutively softer levels, measured with or without a hearing aid. If the Soldier's SRT is no greater than 30 dB HL in the better ear (with or without amplification), an H-3 hearing profile is assigned. (A forceful whisper is spoken at approximately 30 dB HL, while most normal conversation takes place at approximately 50-60 dB HL.) If the SRT is greater than 30 dB HL in the better ear, the hearing profile will be specified as H-4. Acute or chronic ear diseases can result in temporary or permanent hearing profiles. That is, H-3 profiles may also encompass an individual with acute ear disease (tympanic membrane rupture) or a condition that is more chronic in nature (such as a chronic middle ear infection).

This paper identifies some Army military occupational specialties (MOS) that we can be reasonably confident to involve in frequent exposure to high-level impulses from various weapons systems; evaluates the pattern of the hearing profiles for these Soldiers; and considers the appropriateness of relaxing existing military impulse exposure guidelines.


The DOEHRS-HC is a US Government-developed, tri-service "database application" for the management of the Army Hearing Program. Other components of the Army Hearing Program include commercially available audiometers and software to conduct up to eight concurrent hearing screenings and database query software to permit generating reports such as auditing military units for compliance with hearing conservation regulations or deployment readiness status. The results of the hearing screenings are analyzed for hearing loss and/or changes in hearing baselines previously stored in the database, allowing the uniformed or civilian audiologist to counsel the service member regarding noise hazards and hearing protection during annual audiometric screenings.

As noted above, the purpose of this study was to examine the hearing profile status of Soldiers who would be reasonably expected to encounter impulses during their normal duties. These MOSs are presented in [Table 2]. The information extracted from the database did not contain identifiers or any sensitive information potentially harmful to a Soldier if released.{Table 2}

Database queries were made for each year from 1998 to 2007. The queries returned, for each MOS listed in [Table 2], the number and percentage of Soldiers with hearing records in the database for each hearing profile, H-1, H-2, and greater than H-3 (i.e. H-3 and H-4). Also returned were H-3E profiles which have the audiometric configuration of an H-2 profile but are assigned in initial entry medical examinations (i.e. for new recruits only).


In the following eight figures, the proportions of hearing profiles (H-1, H-2, H-3E, and H-3/H-4) are displayed in order in stacked plots. The black solid bars represent the proportion of H-1 hearing profiles, the red bars represent the proportion of H-2 hearing profiles, and the yellow bars represent the proportion of H-3 and H-4 hearing profiles. The H-3E hearing profile, represented by the green bars, represent the audiometric configuration of an H-2 hearing profile but measured during an initial entry physical examination. [23] The line at 0.95 represents the goal of the DRC, protecting all but the most susceptible 5% of the population. Following each figure, a table presents the number of Soldiers included in each of the analyses.

[Figure 2] and [Table 3] present the hearing profile results for Infantryman (MOS 11B) and Indirect Fire Infantryman (MOS 11C). [Figure 3] and [Table 4] present the hearing profile results for Cannon Crewmember (MOS 13B) and [Figure 4] and [Table 5] show the results for Special Forces Weapons Sergeant (MOS 18B). [Figure 5] and [Table 6] show the results for Cavalry Scout (MOS 19D).{Figure 2}{Figure 3}{Figure 4}{Figure 5}{Table 3}{Table 4}{Table 5}{Table 6}

The results for Combat Engineer (MOS 21B) and Heavy Construction Equipment Operator (MOS 21E) are displayed in [Figure 6] and [Table 7]. The numbers pulled from the DOEHRS-HC database for years 1998 to 2002 were very small and are not presented in the figure. The dramatic increase in compliance with annual audiometric assessments from 2003 forward likely resulted from observations of many Soldiers redeploying from Iraq with hearing injuries [24] and subsequent action taken by Army audiologists to assess this problem. The steady increase seen earlier, as shown in [Table 3],[Table 4],[Table 5],[Table 6],[Table 7],[Table 8],[Table 9],[Table 10], was most likely prompted by the increasing emphasis on compliance by the consultants to the Surgeon General of the Army for Audiology and Hearing Conservation and the deployment of the new Army Hearing Program. [25],[26] The numbers of Soldiers returning from Iraq and Afghanistan and the resulting publicity in the press and interest by the Army leadership had a significant impact on compliance to hearing screening regulations. The public press notes that hearing injuries, tinnitus and noise-induced hearing loss, are the Number 1 and Number 2 injuries in Soldiers from service during Operations Enduring Freedom and Iraqi Freedom (c.f., Associated Press as reported in the San Diego Herald Tribune, March 8, 2008).{Figure 6}{Table 7}{Table 8}{Table 9}{Table 10}

The results for the 63 series of MOSs, mechanics, system maintainers, and equipment repairers [Table 2], are presented in [Figure 7] and [Table 8]. The results for transportation MOSs (MOS 88H through 88Z) are presented in [Figure 8] and [Table 9]. Finally, the results for Ammunition Specialist (MOS 89B) and Explosive Ordnance Disposal Specialist (MOS 89D) are presented in [Figure 9] and [Table 10].{Figure 7}{Figure 8}{Figure 9}

Finally, [Figure 10] plots the percentage of H-1 (normal) hearing profiles for each MOS group examined in the present study. The left panel presents the data from the 2003 to 2007 timeframe (the five-year period from the beginning of Operation Iraqi Freedom, OIF) and the right panel presents the five years prior to the start of OIF (1998-2002). Directional Student t-tests were conducted for each mean normal hearing percentage for each MOS for each five-year period. The probability of a Type I error was set at 0.05. The results are displayed in [Table 11] and [Table 12]. For all analyses, only the Special Forces Weapons Sergeant (MOS 18B) during 2003-2007 and Ammunition and Explosive Ordnance Disposal Specialists (MOS 89B, 89D) during both periods MOSs did not show significantly lower rates of normal hearing profiles relative to the 0.95 proportion mandated by DRCs. It is worthy to note that these MOSs had the fewest number of Soldiers included in the database analysis (MOS 18=7635 Soldiers, MOS 89=9798 Soldiers). In contrast, the other MOSs ranged from 37817 data points for Cavalry Scouts (MOS 19) to 256647 records for the Infantryman and Indirect Fire Infantryman MOSs (MOS 11B, 11C). Collapsed across all 10 years pulled from the data base, only the Ammunition Specialist and Explosive Ordnance Disposal Specialist (MOS 89B, 89D) was not significantly below the 0.95 DRC protection target.{Figure 10}{Table 11}{Table 12}


In all but one of the raw analyses, the hearing profile proportions remained relatively constant over the 10-year analyses. In all but one of the 69 proportions presented in [Figure 2] through 9 (i.e. MOS 18B, Special Forces Weapons Sergeant, in 2005), the proportions of Soldiers with H-1 hearing profiles were less than 95%, the de facto risk used in modern noise DRC. In the specialties in which we can reasonably believe that impulses are regularly experienced during training and combat operations, the proportion of Soldiers with H-1 (normal) hearing profiles was as low as 83.6% (MOS 18, year 1999) or 87.4% (MOS 11, year 2003) and rarely even approached the goal of 95%. (The MOS 18B, year 1999 statistic was based on only 287 Soldiers. Therefore, this estimate may be misleading.) In fact, the MOS in which we expect the most frequent routine exposure to impulses (MOS 11) displayed the lowest proportion of Soldiers with normal hearing. Conversely, an MOS which one could assume would have little exposures to high-level impulsive noises seemed to show higher proportions of Soldiers with H-1 hearing profiles (e.g. MOS 89, Ammunition and Explosive Ordinance Disposal Specialists).

The only analysis that showed dramatic differences in hearing profiles in different years was that of the MOS 18B, Special Forces Weapons Sergeant, in which the H-1 (normal) hearing profile increased from approximately 85% to nearly 95% in 2004. The reason(s) for the greater proportion of H-1 hearing profiles in recent years in this MOS are not evident, but it may be the result of different training, tactics, equipment, deployment schedules as well as many additional factors. In any event, while we are not confident of the reasons for the hearing status statistics for Special Forces Weapons Sergeants before 2003, it appears that, for this MOS, the hearing loss statistics from 2004 and beyond are noteworthy and the hearing conservation practices of these Soldiers may warrant further investigation. That is, what are these Soldiers doing right to protect their hearing?


NIHL is a serious problem in the US Army and represents a grave risk to military readiness, survivability, and operational performance on the modern battlefield. Despite calls to relax the MIL-STD-1474D impulse exposure limits, the present analysis suggests that the current exposure limits and/or hearing protection strategies are not sufficiently protective for Soldiers whose occupations can reasonably be expected to include exposures to high-level impulses. The largest group of Soldiers examined in this database analysis are the Infantryman (MOS 11B) and Indirect Fire Infantryman (MOS 11C). While compliance with requirements for annual audiometric assessments has greatly improved in the decade encompassed by this report (increasing numbers by a factor of 10), the proportion of Soldiers with normal (H-1) hearing remained relatively steady, between approximately 87% and 90% (mean = 88.88%; standard deviation = 1.10%).

The purpose of this database analysis was to determine the hearing status of Soldiers regularly exposed to high-level impulses. During garrison or training operations, these impulses are the result of friendly weapons systems and are represented in this paper by the statistics from years 1998 to 2002, before the start of Operation Iraqi Freedom. Given that the goal of protecting at least 95% of the Soldiers in our analyses, we suggest that now is not the time to relax the DRC for continuous or impulsive noise exposures.


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