Causes of Noise-induced Hearing Loss

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Introduction

Hearing loss may be classified differently. The most common classification is based on the pathology where the disease may be conductive or sensorineural. Another classification is based on the cause of the hearing loss, which may be age-related (presbycusis) or in noise-induced cases as discussed below. In conductive hearing loss, the transmission of sound to the cochlea is ineffective, despite the nervous system being intact.

The ear is divided into the external, middle, and inner ear. Obstruction to the transmission of sound along the pathway results in conductive hearing loss. The causes of conductive hearing loss may be congenital such as the meatal atresia, or acquired such as the case of accumulation of wax (Gradwell, & Rainford, 2006).

Sound is perceived in a series of physiological changes that occur in the ear. There is a change of energy from sound to mechanical and then to electrical energy that is transmitted to the brain for interpretation. The main organ that is involved in the sound perception is the cochlea, which has specialized structures that change sound into the nervous impulses.

Sensorineural hearing loss results from lesions that interfere with the transmission of electric impulses from the cochlea to the brain (Orsello, Moore, & Reese, 2013). Lesions in the cochlea result in sensorineural hearing loss.

These cells may be acquired or congenital. Another form of hearing loss is presbycusis, which results from the fixation of the ossicles because of old age (Abel, 2005). A common cause of hearing loss is induced by noise. People operating loud machinery are prone to this disease. The military is a career that is frequently associated with noisy environments, especially where aircraft engines are involved.

According to Azizi (2010, p. 116), Noise-induced Hearing Loss (NIHL) is the irreversible damage to the cochlea hair cells, resulting in partial or total hearing loss. Noisy environments induce NIHL. Some of the mostly affected individuals are the military workers and aviators.

The severity of NIHL varies depending on the duration of exposure and the intensity of the sound that causes the hearing loss. Noise-induced hearing loss can be described as a relatively new condition, if the history of man is anything to go by.

Its origin dates back to just a century when man invented gunpowder and started using it in war. The industrial revolution that followed also contributed to the progression of the condition, with measures of prevention being fronted such as the use of security devices including earmuffs (Dehart, & Davis, 2008).

Protection devices such as the ones mentioned above have a considerable success in preventing noise-induced hearing loss because of the findings of the different studies that were done comparing the development of NIHL in people who used protection as opposed to those that did not (Barney & Bohnker, 2006). However, the use of protection devices has been found to have no effects on the development of presbycusis, which usually develops because of age (Rajguru, 2013).

The noise-induced hearing loss is different from presbycusis, which has an onset that is dependent on age (Dehart, & Davis, 2008). The military is the most affected group by the NIHL due to the reasons stated above, with the aircrew of both civilian and military aircrafts being at a greater risk of the condition. Therefore, this research provides a critical evaluation of the causes of Noise-Induced Hearing Loss (NIHL) in aircrew.

Materials and methods

This research paper was an article review where articles written by researchers after carrying out their research were analyzed. The benefits of using an article research include the relative shorter period required to carry out the research and arrive at the relevant conclusion.

The other reason for an article review is the accuracy of the gathered information since most of the researchers have efficient utilization of resources. The initial step is the development of the study objectives, which involves comparing the different positions in the aircraft in relation to the development of noise-induced hearing loss.

The development of these objectives is followed by the search of relevant articles in the database. The keywords that are chosen for the search include hearing loss, aviation noise, noise-induced hearing loss, aircrew hearing loss, and presbycusis. Pub Med is the database of choice since it provides the best results for the peer-reviewed articles. The search for the key words in the database will provide many useful results, which will be used in the final analysis.

One of the inclusion criteria that are used include the requirement that the articles have to be in English. Those that are not written in English will not be used in the analysis. The different causes of hearing loss will be evaluated in the search, with the search results being scrutinized to see if they have important findings in their references. The selected literature will be used in the research. Some of the important information will be the differences between the diverse types of aircrew.

The search provided over 100 useful literatures. After this search was refined, about 20 articles were found relevant for the study. The assumption was that different members of the aircrew were affected differently by the noise from aircrafts, and hence the need to investigate the extent of the problem in these individuals.

The data from the literature was then analyzed qualitatively, with a determination of significance of the results. Most of the studies were biased on the type of the aircraft operators, with pilots being the main ones under focus.

In one of the studies, Wagstaff and Arva (2009) compared the differences between hearing loss in pilots and air traffic control personnel. In this study, the researchers randomly selected182 medical files from the Civil Aviation Agency. The files belonged to different cadres of aircrew (Wagstaff & Arva, 2009).

The main group consisted of pilots who had different noise exposure as witnessed by the different degrees of noise from the aircrafts they worked with. Helicopter pilots were also selected, with researchers putting the ambient noise in their cockpit at about 90-95 decibels (Wagstaff & Arva, 2009, p. 858). The air traffic control group in this study was mainly selected as a control group, with the study assuming that the group was exposed to the least noise compared to the aircrew.

The use of ear protection during the normal operations was investigated in this research. The method that was adopted to gauge the degree of hearing loss was the use of audiometry, which provided useful audiograms for the study (Wagstaff, & Arva, 2009, p. 858). The obtained audiograms were compared with those obtained for the respective age.

In the research by Wagstaff and Arva (2009), the methods used in the analysis were statistical analysis tests such as the student’s t–test for paired samples. The level of significance used in this study, as in most studies, was set at 0.05 (Wagstaff, & Arva, 2009). This method of analysis was accurate. It provided results that were reliable in most of the analyzed studies.

Results

Most of the studies were comparing the levels of hearing loss between different aircrews while others showed the differences accorded by the use of ear protection. The different types of aircrew that will be discussed in this particular article include the diverse types of aircraft pilots, flight attendants, and the ground crew.

A special category is the space shuttle crewmembers that are also under the risk of developing this kind of hearing loss that is occasioned by the engine noise from the space shuttles. Azizi (2010) asserts that the main occupations that develop hearing loss are the ones that have a considerable degree of noise production.

Such occupations are mainly those working in heavy industries, mining, construction, coal mining, and special forms of farming (Azizi, 2010). In the aviation industry, Azizi (2010) confirms that some of the individuals at risk include the maintenance workers, the flight crews, airline ramp employees, and pilots.

Azizi (2010, p. 117) observes that some factors predispose workers to NIHL. These risk factors include a history of ear disease, hypertension, intake of ototoxic drugs, and smoking (Azizi, 2010, p. 117). Some of the other factors that are known to accelerate or predispose an individual to NIHL include diabetes mellitus, rheumatoid arthritis, chemical substances such as carbon monoxide and hydrogen cyanide, heat, and smoking (Azizi, 2010, p. 117).

Sources of Noise

In the researches that were evaluated, some provided a look at the important sources of noise in the aviation industry. The main sources of noise in the aviation industry that most of the researchers discuss are the aircraft engines. The engines are said to produce the greatest amount of noise.

The evolution of these machines has worsened the problem. According to Miss (2004), the planes used in the era of the World War I and II by the United States’ air force produced as much as 120 dB of noise. This noise hinders communication and/or acts as a catalyst for development of hearing loss.

The invention of the jet engine is said to have made the most important contribution to the problem. Apart from the engine that is the greatest source of noise in the cabin, the other sources of noise in this area include the aircraft conditioning system where the turbulent flow of air causes significant noise (Miss, 2004). The other sources of noise that was evident from these studies include the mechanical and the pressurization systems. Helicopters have special sources of sound generation such as the mechanical and engine systems.

Noise-Induced Hearing Loss in Pilots

The level of NIHL in pilots is said to be higher compared to other aviators. In fact, pilots have the greatest level of hearing loss. In one of the studies that compared the audiograms for military aviators over time, these aircrew members were found to have a considerable degree of hearing loss (Owen, 2004, p. 57).

Owen (2004, p. 57) asserts that there is an expected decrease in the level of hearing with age. The decrease in hearing for the aircrew that was investigated showed a hearing loss that was not age-related in military pilots. The researcher used the results of the scheduled audiometry tests for the military pilots, with the results indicating that the first audiogram was different from those obtained in subsequent years for most of the aviators (Raynal, Kossowski, & Job, 2006).

The audiograms that Owen (2004) obtained for the various generations of aviators showed a major reduction in hearing abilities that were not explained by normal physiology. The number of years that a pilot had flown was also significant in the determination of the degree of hearing loss.

Owen (2004, p. 57) observed that the aviators that had a flown for longer hours had audiograms with higher frequency threshold compared to individuals with fewer flight hours. Therefore, Owen (2004, p. 57) concluded that there was a positive correlation between hearing threshold, age, and the duration of flying for the aviators.

Although the study established that the degree of high frequency hearing loss (noise-induced hearing loss) was greater than what will be normally observed with age, the researcher was unsure of whether the hearing loss was attributable to flying or to other factors (Owen, 2004, p. 57).

In another research that investigated the hearing loss in aviators, the helicopter was the main type of aircraft under focus (Fitzpatrick, 1988). The use of door gunners in the helicopters was recognized as a major source of noise in the cockpit that led to the noise-induced hearing loss in pilots. Fitzpatrick (1988) found out a decrease in hearing level for the aviators in helicopters, with this decline being greater than 5dB that they used as the age-corrected hearing loss (Fitzpatrick, 1988).

Most of the aircrew that used helicopters satisfied the criteria used by Fitzpatrick (1988) to evaluate hearing loss, with a hearing loss increasing fourfold in the aviators using the Bell 412 helicopters. However, the material used in this research was small to make a considerable conclusion. The researchers suggested the use of more materials for better conclusion making (Fitzpatrick, 1988).

In a study by Wagstaff and Arva (2009), the researchers investigated the differences in hearing loss for pilots and the air traffic controllers. This study used a larger population of subjects.

Hence, the results can be used to make accurate conclusions. The researchers divided their participating aircrew into helicopter pilots, airline pilots, and the Air Traffic Control personnel in an effort to investigate the different audiograms that these aviators had over several years. Just like in the previous studies, the researchers established that the age was a major cause of hearing reduction.

Therefore, they applied age correction for the subjects (Wagstaff, & Arva2009, p. 860). The researchers observed a significant change in the level of hearing for the different aviators. While using the ATC personnel as the control subjects, the group that was largely affected by the hearing loss was the civilian pilots (Wagstaff, & Arva 2009, p. 860). The pilots reported a considerable degree of hearing loss that was attributable to their occupation.

Some of the studies investigating the degree of hearing loss in pilots did not find a significant degree of reduction, with the explanation being the better practices used to protect these individuals in their respective places of work (Kuromen, Toppila, Sorri, Paakkonen, & Starck, 2004). The researchers observed that the physical conditions of the pilots that they used in their study were perfect. They had normal hearing (Kuromen, Toppila, Sorri, Paakkonen, & Starck, 2004).

The factors that were established as relevant in the prevention of hearing loss for these pilots in the military include the health monitoring that is provided by the military and the frequent examination for these individuals (Kuromen, Toppila, Sorri, Paakkonen, & Starck, 2004).

The possible causes of hearing decrease according to the researchers included the noise from the guns mounted on the military planes and the aviation noise. The model that was used is the Noise Scan Model that is established as a useful one in the assessment of hearing loss in military aviators (Kuromen, Toppila, Sorri, Paakkonen, & Starck, 2004).

A comparison of different types of aircraft pilots was made in several studies, with most of the studies showing significant correlation between the type of aircraft that was investigated and the degree of hearing loss (Kuromen, Sorri, Muhli, & Paakkonen, 2003).

The aircraft that were found to have the highest reported incidence of NIHL in pilots is the jet-powered type of aircraft where the researchers found a decrease in hearing ability that was not related to age. This observation brings about the difference between age related hearing loss (presbycusis) and the noise-induced hearing loss that is evident in noise operating environments such as the aviation industry.

The decrease in hearing for the aircraft pilots was investigated in other studies that sought to see the difference between hearing prior to take-off and after landing (Kuromen, Sorri, Muhli, & Paakkonen, 2003). A specific study that sought to investigate the differences in hearing threshold after a flight was done in Finland where aviators had audiometry done before take-off followed by another one after landing (Kuromen, Sorri, Muhli, & Paakkonen, 2003).

The results of this study showed insignificant hearing loss for the subjects, with the suggested reasons being the protection accorded by the various gadgets (Kuromen, Sorri, Muhli, & Paakkonen, 2003). However, the researchers stated that the frequent monitoring of the hearing capabilities of aviators in the Finnish air force was necessary to interpret for future studies to make better conclusions (Kuromen, Sorri, Muhli, & Paakkonen, 2003).

Other Aviators

In the present age of space travel, a new source of noise is the space shuttle, which produces considerable noise levels during takeoff and in flight. Some of the studies that were used in this particular study mentioned space shuttles as some of the crafts that may predispose people to hearing loss (Fay, & Popper, 2011). One of the limiting factors in the estimation of the hearing loss from the space shuttles is the number of studies that exist in this area.

Few researchers have looked at hearing loss specifically in aviators who are involved in space travel. It is also hard to evaluate hearing loss from spacecrafts since the numbers of flights made by each of the aviators in a lifetime are not many. If any hearing loss is observed in the space shuttle, it will also be difficult to make a correlation between this hearing loss and the activities in the shuttles because of the few flights (Fay, & Popper, 2011).

Space shuttle crew is involved in training. This practice predisposes them to NIHL (Fay, & Popper, 2011). Despite this condition occurring in the shuttle crews, one cannot make a conclusion that the NIHL is from the space shuttles or from the training activities. However, it is postulated that space shuttle aircrew members are in greater danger of developing hearing loss occasioned by the frequent space travels and the conditions in which they train.

Some of the other aviators that the research focused on include the helicopter pilots. They were established as a special group of aviators. In most of the studies that focused on helicopter pilots and the prevalence of NIHL among this population, the researchers applied the same methodology to get to the results. The findings indicate that the helicopter is one of the machines that produce significant noise during flight. Aviators that operate it are also likely to develop NIHL (Dehart, & Davis, 2008).

Unlike other types of aircraft, the helicopter has a different method of propulsion, which is located above a significantly small-sitting space. The noise produced by the engine that is used to turn the rotors is significantly great. Pilots are constantly exposed to this noise. Apart from the avionic noise in helicopters, some of the other extra sources of noise in these machines include the weapons that are mounted on it in the military and the on-board electronic gadgets (Dehart, & Davis, 2008).

In most studies, helicopter pilots are said to have the highest exposure to noise in the aviation industry, with a greater number of them developing NIHL as compared to other aviators (Fitzpatrick, 1988). Despite the finding of increased susceptibility for helicopter pilots to aviation noise in most of the researches, some studies concluded that there was no difference between the hearing loss in these aircrafts and that from other aircraft pilots (Dehart, & Davis, 2008).

Ground Crew

Most of the studies investigating the prevalence and presence of hearing loss in the aviation industry used the data obtained from pilots (Von Gierke, & Kent, 1981). Most of the studies had a basic underlying assumption that the pilots are the main individuals affected by the noise from the aircrafts.

The ground crew members also face the challenge of high frequency and loud noise since they work in an environment that is full of the same (Von Gierke, & Kent, 1981). Most of these individuals have different areas where they work, including air traffic control, engineering, flight management, and other complementary roles such as luggage control and security. Although they work in different departments in the airports, they are also susceptible to the loud noise coming from the engines of aircrafts (Fitzpatrick, 1988).

In one of the studies that investigated the prevalence of hearing loss in the ground crew members, the age-adjusted hearing levels for these individuals was not different from that observed in the general population (Von Gierke, & Kent, 1981). A general reduction rate in hearing was observed, with this level varying with the areas in which the individuals worked (Von Gierke, & Kent, 1981).

However, the hearing loss was present in some of the individuals working in departments such as aircraft maintenance and engineering departments (Rajguru, 2012). In the members of the aircrew that were not involved in flying, the use of protective gadgets was recognized as the main protective measure that helped them prevent the decrease in hearing ability.

NIHL and Ear Protection

One of the factors that were considered important in influencing the results of the studies listed above is the use of ear protection in the individuals mentioned in these studies (Rajguru, 2012). The use of noise protection gadgets in aviators is also the subject of this study. It investigates the different types of protection showing how each will alter results of the studies.

This research also sought to investigate which device between earmuffs and earplugs is more effective in offering protection to aviators from NIHL. Miss (2004) is one of the researchers who investigated the use of different methods to reduce the effects of noise in the aviation industry on the aircrew. He states a need to adhere to noise reduction criteria, and that different methods of noise problem alleviation were already in use (Miss, 2004).

The most commonly used method of reducing the effects of noise on aircrews is the use of a flight helmet. According to Miss (2004), this headset approach is effective in most cases. The researcher proposed new forms of helmets that would be even more effective in noise reduction.

However, the main disadvantages to the use of these gadgets include their weight and sizes, which are a problem for pilots (Miss, 2004). Another form of noise reduction is the use of devices that reduce noise actively (Miss, 2004). According to Miss (2004), this method is also known as Active Noise Reduction (ANR).

Although the use of ANR in noise reduction is thought to be more effective compared to other methods of noise reduction in the cabin, the practical application has been a challenge, with few aviators and aircrafts having the capabilities to use the devices (Miss, 2004). A comparison of the different types of helmets that Miss (2004) discussed is possible. It shows that the ANR helmets are better at reducing noise compared to the standard helmets.

Role	Std Helmet	ANR Helmet mar 99	ANR Helmet Apr 00
Pilot	89.3 (4.3)	85.6 (3.5)	80.9 (2.8)
Observer	90.4 (3.9)	83.8 (4.5)	80.7 (4.4)

Mean noise dose (and associated standard deviations) measured in Sea King AEW2 for Standard and ANR Mk4 flight helmets

Source (Miss, 2004, p. 13)

Earplugs

The use of earplugs for the protection of individuals from sources of destructive sound is common. Earplugs have been associated with considerable degrees of success (Abel, & Odell, 2006, p. 899).

There are different makes and models of earplugs. This fact makes the differences in attenuation of sound evident in the users of the devices (Abel, & Odell, 2006, p. 899). Abel and Odell (2006, p. 899) are some of the researchers who conducted investigations into the use of earmuffs and earplugs to protect aviators and other individual from NIHL. These researchers stated that the use of earplugs might provide better protection for individuals, especially pilots (Abel, & Odell, 2006, p. 899).

Abel and Odell (2006, p. 899) confirm, “In general, earplugs provide relatively more attenuation (15–40 dB) below 1 kHz, but are about the same above 1 kHz for highly rated devices.” Some of the factors that affect the attenuation of these devices are dependent on the individual user.

The main difference was in the ability to fit the devices well (Abel, & Odell, 2006, p. 899). Some of the other factors that Abel and Odell (2006, p. 899) established as affecting the attenuation offered by earplugs include the maintenance for the devices, the sizing that is done and the headband tension (Abel, & Odell, 2006, p. 899). The combination of earmuffs and earplug should offer better protection for loud noises according to Abel and Odell (2006, p. 899).

Earplugs offer mechanical protection to loud noises as compared to other forms of protection such as ANR. Therefore, they may be uncomfortable for users based on the differences in anatomies for the users. The ‘make’ of the earplug is also important as different companies make different models based on the standard ear shape for their region and expected market (Abel, & Odell, 2006, p. 899).

Earmuffs

Earmuffs are considerably bigger that earplugs. They are associated with more comfort for the users. However, a key difference is the level of noise reduction and protection that each of these accord to the users. According to Abel and Odell (2006, p. 899), “For earmuffs, attenuation increases from about 15 dB at 0.125 Hz to about 35 dB at1 kHz and then remains fairly stable.” Earmuffs are important [protection gadgets for pilots and other individuals exposed to high levels of noise.

Many studies have found these gadgets to be useful in the reduction of risks associated with a loud noise. These instruments have been used in reducing noise from weapon systems. Abel and Odell (2006, p. 899) assert, “Studies of measurements made inside earmuffs with probe microphones have shown that the attenuation may be sufficient to protect against a pistol or rifle shot, but will not reduce the level of blasts from a bazooka or canon to safe levels.”

Earmuffs offer considerable protection from noise related to aviation, but are not as effective as the use of earplugs in the protection of individuals in these activities. In a study comparing the effectiveness of using the muff and the plugs alone, the researchers found the muff to be less effective compared to the plug-in reducing exposure to noise in individuals (Abel, & Odell, 2006, p. 903).

However, they observed that the combination of both devices was more useful in reducing the noise exposure to the ears (Abel, & Odell, 2006, p. 899). Therefore, these researchers suggested that the devices consisting of earmuffs and earplugs combined into one device were more useful in preventing NIHL as compared to other types of devices, or when the two are used alone (Abel, & Odell, 2006, p. 901).

Attenuation (dB) as a function of frequency: Effect of ear condition

Source (Abel, & Odell, 2006, p. 901).

Discussion

This study finds that the problem of Noise-Induced Hearing Loss is a common one. The group that is significantly affected is that of the aviators. This group is frequently exposed to noise levels that are above the daily average for other occupations and professions (Dehart, & Davis, 2008).

The different jobs that are available for aircrew members predispose them to the NIHL. The profession that is mostly affected is the piloting part (Dehart, & Davis, 2008). The evolution of NIHL is found to be different from presbycusis in pathophysiology. The present structural abnormalities differ between the two groups of patients (Rajguru, 2012).

The aviators that are mostly affected by NIHL are the helicopter pilots because they operate crafts that have many sources of noise (Rajguru, 2012). However, some of the studies found that there was no difference between the occurrences of NIHL in helicopter pilots from that observed in other aviators.

The underlying finding in most of these studies is that the aircrew members have a higher chance of getting NIHL in their lifetime as compared to other individuals. Apart from the development of NIHL in pilots, the other group of aircrew members that the study finds to be at increased risk of developing NIHL is the ground crew.

According to Kuromen, Sorri, Muhli, and Paakkonen (2003), ground crew individuals are exposed to different sources of noise, with the main source being the aircrafts that they service and maintain. The development of NIHL in the ground crew is not as pronounced as in pilots. The reasons behind this finding include the avid use of protection for the individuals and the different areas where they work.

The other type of aircrew that the research investigated as being predisposed to NIHL included the space shuttle aircrew people who are exposed to considerable noise levels (Fay, & Popper, 2011). The studies evaluated were not rewarding, with most citing the fewer times that these crew members are exposed to noise in their few space travel missions (Fay, & Popper, 2011). The use of ear protection is recognized as important in the mitigation of the effects of loud noise in aviators in the form of NIHL.

The two types of protection mainly used are the earplugs and the earmuffs, which have shown effectiveness in reducing the noise exposure in aviators (Abel, & Odell, 2006, p. 903). This study finds that the use of earplugs is more useful in the protection of aviators and other aircrew members to the development of NIHL. However, the combination of the two methods of noise protection is more effective than the use of any of them alone (Abel, & Odell, 2006, p. 903).

The study qualified as an article review by using findings from previous studies. There is a need to carry out researches that are prospective, which will follow the progression of hearing in aircrew members working in noisy environments. There is also need to carry out studies on the prevalence of NIHL in space shuttle aircrew, as one of the limitations for this study is that there were few researchers looking at NIHL this particular group.

Conclusion

This study evaluated the findings of some of the researchers who conducted studies on the prevalence, cause, and differences of Noise-induced Hearing Loss in different aircrew members. The findings indicate that working as an aircrew member in different capacities predisposes one to NIHL.

Pilots are specifically at greater risk of developing NIHL, with the reasons behind this observation being provided. The study also concludes that the use of earplugs is better in protecting against the effects of loud noises. However, the application of both earplugs and earmuffs is better in achieving the same results.

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