Between and , the army acquired its first six military audiologists. They were not used to implement and enforce hearing conservation standards, however, but instead worked in army medical centers performing clinical duties. It was not until that 25 additional army audiology positions were added to the inventory. These new audiologists spent only half of their time working in hearing conservation and the other half in the clinical setting; nevertheless, their impact was astounding.
Figure 2 shows a significant decrease in hearing loss in the US Army over time that is directly attributable to the hearing conservation efforts of the new audiologists. Percentage of combat troops with acceptable hearing, by length of time in the army. These new army audiologists recognized that they were facing serious obstacles in implementing hearing conservation, including bureaucratic red tape, the lack of formal hearing conservation education in their audiology programs, a slowly changing military culture, and a lack of standardization of hearing conservation programs at individual installations.
In , to facilitate collegiality and professional development among military audiologists, an organization called the Military Audiology and Speech Pathology Society now known as the Military Audiology Association was formed. It was instrumental in the future of hearing conservation because it provided a foundation for the standardization of military hearing conservation programs and a way to mentor and educate audiologists with little or no hearing conservation experience.
That number did not include compensation for hearing loss with a concurrent disability or cost of hearing aids, batteries, or repairs. Occupational Exposure to Noise. If the intensity of an exposure increases by 5 dB, then the dose doubles. The dose of noise exposure determines how much time an individual can safely be exposed to hazardous noise. These standards would be an initial benchmark to hazardous exposure levels in the military. Even with the quantifying of hazardous noise exposure and laws to enforce the standards, there was still no system in place to capture hearing conservation audiometric data and to measure compliance.
In , the General Accounting Office now the Government Accountability Office released an investigative report on government working conditions. Further, the report requested that Congress amend the Occupational Safety and Health Act to bring federal agencies under the inspection control of the Department of Labor.
This document provided guidance and requirements for implementing hearing conservation. To implement DODI In the same decade that the Internet was introduced to the public, the military's goal of computer-automated data capture became a reality. In , an effort was made to increase compliance of occupational health screening by combining the services into a mobile vehicle called the military occupational health vehicle MOHV.
Subsequently, the army purchased 16 of them for use worldwide. This development facilitated the promotion of the newly developed HEARS by taking the monitoring equipment and services to the soldiers, a convenience that commanders favored. Although comprehensive occupational health screenings were the goal, the MOHV was primarily used for hearing monitoring, the other health services preferring that screenings be conducted in a fixed facility.
By January , over half a million allied troops were deployed in Saudi Arabia and throughout the Gulf region to liberate Kuwait from the Iraqi invasion.
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The allied armies launched the ground war on February 23; by March 31, Iraq accepted the terms of a ceasefire and the allied troops began to be sent home. During the 2-month redeployment process, 29 hearing screenings and comprehensive audiometric follow-up evaluations were conducted. A manpower resource model estimated that 82 weeks work hours of postdeployment audiometric evaluations were saved by providing the service in Kuwait while the soldiers waited to fly home.
Unfortunately, funding for the MOHV concept was not sustained after the military drawdown of the early s. Acceptable hearing is defined as when the pure tone average Hz, Hz, and Hz is no worse than 30 dB in each ear, with no individual threshold greater than 35 dB and thresholds not exceeding 55 dB at Hz. Whether in peacetime or wartime, hazardous noise is one of the primary occupational hazards in the army, and the risk of soldiers incurring noise-induced hearing loss is greater than it has been in over 30 years.
This is a result of current combat operations, increased numbers of combat soldiers, extended periods of weapons training, and the deployment of new and more powerful noise sources from weapons systems, vehicles, and aircraft. US forces in Iraq and Afghanistan have experienced a substantial number of blast injuries from improvised explosive devices, rocket-propelled grenades, and mortar rounds. The combat arms earplug was introduced into the military at the start of the war in Afghanistan Operation Enduring Freedom.
The device allows soft sounds to flow unimpeded through a filter but blocks loud impulse sounds, such as an explosion or a rifle discharging. This allows effective communication, enables situational awareness, and provides protection from hazardous weapons firing and explosions.
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All deploying soldiers were therefore issued the earplugs in In fact, the US Marine Corps was so convinced of the effectiveness of the combat arms earplug that it ordered over 20 pairs, thereby temporarily depleting the entire national stock in During the first year of the war in Iraq, an average of one soldier a day was medically evacuated for complaints related to hearing loss. Consequently, recommendations were made in October for an audiology support program that would use a minimum of 9 hearing conservation technicians assigned to locations of dense troop population.
To curb the need for medically redeploying soldiers, it was recommended that one army audiologist serve as a consultant, examine follow-up patients, assign duty limitations, dispense hearing aids, verify threshold shifts, and evaluate any possible pathology identified by the hearing monitoring.
Initially, only one audiologist with no support staff was authorized, who used old equipment that had been donated by a clinic in Landstuhl, Germany. Also during the first year of the Iraq war, a study was conducted in army medical facilities comparing hearing loss among soldiers who had been exposed to combat in Afghanistan or Iraq and among those who had not. April 1, , to March 31, Postdeployment troops are soldiers who had recently returned from active duty in Iraq or Afghanistan; nondeployed troops are soldiers who had not served in combat.
The hearing conservation paradigm had shifted, and it now had to consider the soldiers on the battlefield. As a result, a restructuring occurred, and a contemporary model called the Army Hearing Program was born. The Army Hearing Program is charged with preventing noise-induced hearing loss in soldiers and ensuring their maximum combat effectiveness in training as well as during deployments.
To accomplish this mission, four pillars of service were established: The program aims to maintain a high state of readiness and to protect hearing without compromising the effectiveness of the soldier. A study evaluating the importance of hearing for soldiers in combat was conducted at the US Army Human Engineering Laboratory, which investigated the impact of noise and other variables on the mission effectiveness of tank crews.
As shown in Table 2 , poor understanding led to slower response times, which can mean the difference between life and death on the battlefield. Communication in a tactical environment is of utmost importance. TCAPS compose a new category of electronic hearing protection that uses active noise reduction to soften noise and enhance speech discrimination while at the same time reducing noise by up to 40 dB. In addition to being light and rugged, TCAPS provide protection and let soldiers monitor environmental sounds, communicate, accurately gauge auditory distance, and localize sound sources without hindrance.
Further, the devices allow radio connections specifically used by the military to be processed without interrupting the signal when the TCAPS are actively blocking environmental sounds. Although this category of device is still being studied and protocols for use are being created, it represents a new era in the history of hearing protection. Current data show that The implications for the army are great.
When soldiers reach these levels of hearing loss, they must be evaluated for the ability to perform their duties safely and effectively. Depending on the findings, they may be given the option of changing to a job that does not put their hearing at further risk or leaving the service with a medical discharge.
In light of this, 10 much-needed army audiology positions were added in These positions will have a positive impact on the Army Hearing Program, but there will still be only two thirds as many army audiologists as there were before the military drawdown of the early s. Still, with war, there are some injuries related to noise-induced hearing loss that cannot easily be prevented, such as traumatic brain injury, dizziness, auditory neuropathy, and central auditory processing disorders.
Evaluation and management of these injuries are also within the scope of the practice of audiologists and further strengthen the need for adding more military audiology accessions to support the war effort. In , the number of new applicants granted primary disabilities for hearing loss and tinnitus was 49 and 61 , respectively. Tinnitus was responsible for the largest number of primary disabilities in , followed closely by hearing loss.
Prevention, however, is still the best answer not only from a cost-benefit standpoint but for the quality of the lives of veterans and their families. The Army Hearing Program represents a means to eliminate hearing loss as a result of battlefield conditions in the 21st century. Some brain injuries may also come from occupational hazard exposure due to blast pressure from firing heavy weapons during training.
There are multiple ways in which blasts can injure the brain. The primary mechanism of injury is the wall of blast pressure from an explosion. This blast wave can travel faster than the speed of sound, up to 1, feet per second, causing sudden changes in pressure up to 1, times greater than atmospheric pressure. Fires, toxic gases, burns, or crashes can follow quaternary mechanism.
The non-primary mechanisms of injury are relatively easy to understand and protect against through traditional means such as body armor, helmets, fire-resistant uniforms, etc. The primary mechanism of injury — the blast pressure wave itself — is less understood. Additionally, these studies have shown that multiple exposures lead to a greater likelihood of injury. Effectively, this translates to a lower threshold of injury following multiple exposures.
Studying these injuries is challenging for many reasons. In fact, a military study of TBI found that military doctors often did not account for harm that did not result in bleeding or a penetrating injury. Soldiers could belatedly experience injury symptoms without an obvious corresponding blast event in their past to explain them. Cognitive and mood challenges can persist and overlap with those of PTSD, complicating diagnosis.
One cause for the rise in TBI cases in recent conflicts may be that soldiers are surviving wounds that would have been fatal in previous conflicts. As battlefield medicine and body armor have improved, the ratio of wounded soldiers to killed soldiers has increased since Vietnam. In fact, one research paper identified the improvements in personal protective equipment as the direct cause of increased numbers of TBI, since armor now protects the torso and lungs from blasts that previously would have been lethal. It is also possible that brain injury may come from repeated blast exposure from soldiers firing heavy weapons.
The Army has taken steps to better understand the link between blast overpressure exposure and TBI. To this end, DoD has developed blast monitoring devices, also called blast gauges, to measure the blast overpressure to which servicemembers are exposed. An ideal device could objectively assess in the field whether a soldier has brain injury, analogous to a thermometer assessing whether an individual has a fever.
Blast gauges are environmental sensors, however. They cannot and are not intended to perform this function. Other emerging technologies may yield portable gauges that could directly assess any cognitive deficits in the field. In this sense, they are analogous to radiation dosimeters, which measure exposure to radiation levels. Blast monitoring devices serve several purposes. One, they can be used as screening devices to identify soldiers who may have been exposed to high levels of blast-related stress pressure, acceleration, and temperature and should be seen by a medical provider.
They are not a diagnostic tool for assessing brain injury and cannot take the place of a medical professional, but they can help identify soldiers for screening. When used as an objective screening tool, blast gauges can be helpful in guarding against soldier under-reporting of TBI. Two, blast gauges can be used to identify tactics and techniques during weapons firing and combat that could mitigate soldier exposure to blast pressure. Three, with additional data, blast gauges potentially could be used to estimate the probability that a soldier has suffered a brain injury due to primary blast pressure exposure.
This would be analogous to a radiation dosimeter, which can be used to help estimate the likelihood of injury from radiation exposure. For example, it is not known whether there is a single threshold for injury or a risk curve, where the probability of injury increases with higher-pressure exposure. It is also not known what the effects are of repeated low-level exposures. Animal studies have linked higher levels of blast pressure exposure with a greater likelihood of injury and greater severity of injury, and multiple exposures with a greater likelihood of injury.
More data is needed, however — particularly of human exposure to blast pressure — to better understand the relationship between blast pressure exposure and brain injury. Blast gauges are an essential tool for collecting this data. Blast gauges are needed to help build a comprehensive database of incidents where soldiers have been exposed to blasts, the recorded blast pressure, and any cognitive deficits afterward. This is particularly the case if low-level blast exposure leads to cumulative effects over time. It is also essential that the Army record data from enemy-initiated blast events, since they will result in different pressure signatures than heavy weapons firing.
The combat environment is less controlled than the training environment, a condition that invariably complicates data collection and analysis, but training cannot and should not perfectly simulate the hazards soldiers are exposed to in combat. In order to refine its understanding of blast-induced brain injury, it is essential that the Army collect data for any combat exposures to blast overpressure.
The Army should expand on existing blast pressure monitoring in training and establish a longitudinal medical study on blast pressure exposure during combat and training in order to better understand the relationship between blast pressure exposure and brain injury. In parallel, the Army should conduct a blast surveillance program to monitor, record, and maintain data on blast pressure exposure for any soldier, in training or combat, who is likely to be in a position where he or she may be exposed to blasts. The Army should include brain imaging of soldiers who have been exposed to blasts as part of their medical study to better understand how blasts affect the brain.
High-quality brain imaging can give quantitative measures of changes within the brain, even at lower levels of blast exposure than may be detected by a neurological exam. Because primary blast-induced TBI often does not result in outward evidence of injury, it is difficult to pinpoint the direct injury mechanism. Modeling how the blast wave travels through the tissue is challenging, requiring complex and detailed computer models that are hard to correlate with experimental data. Experiments are challenging, since the threshold of injury cannot be directly translated from animal experiments to humans.
The diversity of sizes and shapes of animal skulls affects how blast waves propagate through the head into the brain. Finally, in addition to the current poor understanding of how blast waves affect the brain in general to determine injury thresholds, the possibility of aggregate damage from many small exposures, each making the brain more susceptible to future injury, multiplies the possible blast exposures researchers must study to evaluate possible correlations to injury.
There are competing theories for how blast pressure causes primary brain injury. Initial analysis assumed blast waves injured the brain in a similar manner to concussions, which result in brain injury due to the rapid acceleration and deceleration of the head, forcing the brain against the skull. Various theories for injury mechanism for primary blast waves, and resulting implications for helmet design, include:. Research into the possible injury mechanisms of blast should be continued.
The Army should accelerate computational modeling and experimental research, including with large animal models, into primary blast wave injury in order to better understand how blast overpressure damages the brain. These efforts should work in tandem with neurologists to improve understanding of TBI and move to refine a theory of injury over time. Army helmets already include requirements to protect against ballistic and blunt impact injuries secondary and tertiary injuries.
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There is currently no requirement, however, for protection from primary blast pressure waves. Research has demonstrated that modifications to helmet designs change the strength and distribution of blast waves inside the head. This research suggests that some helmet designs may be able to mitigate the blast pressure transmitted to the brain, although further research is needed. The increased pressure is also coupled with movement of the head relative to the helmet, increasing the mechanical load that could cause injury from skull flexure consistent with injury from impacts.
While the head still experienced overpressure, it was significantly less than without a helmet. Most significantly, this study found that adding a face shield in computer simulations reduced the blast pressure in the brain by 80 percent. The study considered wave reflections from interactions with the body, as well as different blast locations to understand the variety of pressure impacts on the skull.
However, adding mandible protection increased peak pressure in the forehead due to the wave reflections within the helmet. It is possible that existing commercially available helmet designs could reduce the blast pressure transmitted to the brain. These helmets pose tradeoffs due to the increased weight, as well as increased torque on the neck. As with body armor, additional weight on the head could diminish situational awareness and operational effectiveness for the user. Face shields could also obstruct vision.
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Interoperability with existing night-vision devices and weapons optics could also be a concern with full-face helmets, as well as object detection and ability to rapidly engage targets. Nevertheless, the potential may exist to reduce soldier exposure to blast pressure waves based on readily available off-the-shelf solutions. The Army should test existing helmets, including commercially available variants with modular mandible and face shields, to determine which configuration and materials best protect against primary blast wave injury as a near-term mitigation against possible brain injury.
Helmets should be tested for their ability to decrease blast overpressure from a variety of angles and directions, as well as for different types of blasts — explosive blasts as well as from firing heavy weapons such as artillery, recoilless rifles, or. The same type of helmet may not equally protect from both types of blast. The Army should then conduct a tradespace study of the various helmet designs in order to compare the amount of reduced blast pressure to any negative effects, such as increased weight and torque on the neck, reductions in situational awareness, and other operational effectiveness metrics.
The Army is at a crossroads in soldier protection against primary blast-induced brain injury. Helmets are not currently developed with blast injuries in mind, as there is no blast injury requirement. The Army could continue down the current path with no requirement for helmets to protect against blast pressure.
The rationale for this course of action would be to wait until the mechanism and threshold of injury for blast pressure waves is more fully understood before creating a requirement. This is the most cautious path from a scientific perspective but the riskiest path for soldiers. It would leave soldiers exposed to potentially harmful blast pressure waves with whatever incidental protection the ACH and IHPS provide.
These helmets likely provide some protection against primary blast wave injury relative to no helmet, although their level of protection will almost certainly not be optimal if they are not designed to do so. An alternative is to establish an interim requirement for protection now, based on what is known, and adjust the requirement over time as the science matures.
Fully understanding the causes of blast injury could take years or even decades, but the Army has the opportunity to begin protecting soldiers today. The Army should take this opportunity. Based on the results of the helmet tradespace study demonstrating the ability of certain designs to reduce overpressure exposure and the drawbacks of various designs, the Army should establish an interim requirement for protection against blast overpressure while continuing further research to refine the requirement over time.
By instituting a requirement, the equipment community has a metric to target when designing protective equipment, and the broader commercial and scientific communities have a baseline from which to improve. Most importantly, a protection requirement will institute some level of protection today, balanced against other tradeoffs such as weight, as best as is possible given the state of the science.
Over time, research and experience may demonstrate that the requirement is too high or too low — or perhaps focused on the wrong metric — and it can be adjusted accordingly as the science matures. Emerging evidence suggests that soldiers may be exposed to significant levels of blast overpressure when firing heavy weapons, such as the Carl Gustaf recoilless rifle, even in training. Animal studies have shown that multiple exposures lead to a greater likelihood of injury. Effectively, this translates to a lower threshold of injury following multiple exposures, 78 meaning even low-level exposures can have cumulative effects.
Cumulative effects from repeat blast exposure can even occur across multiple days. In DoD studies, servicemember cognitive deficits following heavy weapons firing took 72 to 96 hours to resolve to a level at which they were not statistically significant. A single daily exposure delivered for three consecutive days resulted in percent greater physiological change, suggesting there is a cumulative effect of the exposures and that a rest period of 24 hours is not sufficient to fully recover.
The long-term effect of prolonged exposure to repeat sub-concussive blast events is unknown, but there is some evidence to suggest concern.
A Army survey compared concussion and post-concussion associated symptoms in breachers, who are exposed to repeated low-level blasts, to non-breachers. Additional research is needed on the long-term effects of low-level blast exposure, particularly from shoulder-fired weapons. The Army should expand ongoing studies of blast exposure in training to include all soldiers who are exposed to high overpressure weapons e.
In order to obtain objective measurements of blast exposure, the Army should require all soldiers to wear blast gauges when training with high overpressure weapons. Blast gauge measurements should be recorded as part of the blast surveillance program, in parallel to a longitudinal medical study on blast pressure exposure and brain injury. The cumulative effect of prolonged exposure to low-level blast events may take years to manifest.
Accurate records of soldier blast exposure, including during training, are essential to improving understanding of blast-induced brain injury. One particular concern is that mild TBI may worsen over time. Understanding the long-term effects of low-level blast exposure will inevitably take years. There are concrete steps that the Army can take today, however, to improve soldier safety during training. All Army shoulder-fired heavy weapons have limits on the number of rounds that can be fired in a hour period, depending on the firing position sitting, standing, kneeling, or prone.
The blast overpressure experienced by soldiers depends on the firing position because of reflections from the ground. According to Army manuals for shoulder-fired weapons, these limits exist because of hazards from overpressure and noise. With the exception of the AT4CS, which is specifically designed for firing in confined spaces, the limit on firing any of these weapons is no higher than six shots per day.
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For some weapons and firing positions, it may be as low as zero shots in training. Firing restrictions on the M72 LAW four shots per 24 hours also apply to any soldiers within a meter radius of the weapon. Limits on firing some shoulder-fired weapons, such as the AT4, have existed for over 20 years, but these limits have evolved over time. Emerging evidence from DoD studies on blast exposure suggests that current firing limits may fall short of adequately protecting soldiers in significant ways:.
Firing limits should be revised downward to a level such that allowable exposures are not associated with cognitive deficits after firing. Additionally, firing limits should cover exposures across a longer time period, on the order of 72 to 96 hours.
Firing limits should include a minimum safe distance for observers and instructors, and should account for the possibility of multiple types of heavy weapons being fired in a single day. Additionally, the Army should establish cumulative annual and lifetime limits for blast exposure in training. Standards are ineffective if they are not enforced, however.