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   Abstract
   Introduction
   Methods
   Results
   Discussion
   Acknowledgment
   References
   Article Tables
 

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  Table of Contents    
ARTICLE  
Year : 2011  |  Volume : 13  |  Issue : 50  |  Page : 64-70
Occupational exposure to noise and the prevalence of hearing loss in a Belgian military population: A cross-sectional study

1 Unit of Epidemiology and Biostatistics, Staff Department Well Being, Bruynstreet 1, 1120 Brussels, Belgium
2 Institute of Statistics, Biostatistics and Actuarial Sciences, Université Catholique de Louvain, Voie du Roman Pays 20, 1348 Louvain-La-Neuve, Belgium
3 Center of Medical Expertise of Otolaryngology, Queen Astrid Military Hospital, Bruynstreet 1, 1120 Brussels, Belgium
4 Occupational Health and Safety, Staff Department Well Being, Bruynstreet 1, 1120 Brussels, Belgium

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Date of Web Publication15-Dec-2010
 
  Abstract 

The armed forces are highly exposed to occupational noise. The aim of this study was to evaluate the prevalence and noise exposures associated with the severity of hearing loss (HL) in a Belgian military population. A cross-sectional study was carried out at the Centre for Medical Expertise (CME) and in four Units of Occupational Medicine (UOM). Hearing thresholds were determined by audiometry. The examination included a questionnaire on hearing-related medical history, and noise exposure in military and leisure time activity. A multinomial logistic regression model was used to assess the association of the severity of HL with tinnitus, with the military occupation, and with noise exposures. Of the 2055 subjects aged 18-55 years, 661 (32.2%) had a slight HL (25-40 dB), 280 (13.6%) had a moderate HL (45-60 dB) and 206 (10.0%) had a severe HL (>60 dB) of 4 and 6 kHz for both ears. The prevalence of slight, moderate and severe HL increased significantly with age and was higher for subjects from Paracommando and infantry units. Fighting in Built-Up Area (FIBUA) training, shooting with large caliber weapons, and participation in military exercises were the best determinants of HL in this population. These results suggest that subjects from infantry and Paracommando units run the highest risk of HL because they are exposed to very loud noises in their professional life, like large caliber shooting and FIBUA training.

Keywords: High frequency hearing loss, occupational noise, military personnel, prevalence

How to cite this article:
Collee A, Legrand C, Govaerts B, Veken PV, De Boodt F, Degrave E. Occupational exposure to noise and the prevalence of hearing loss in a Belgian military population: A cross-sectional study. Noise Health 2011;13:64-70

How to cite this URL:
Collee A, Legrand C, Govaerts B, Veken PV, De Boodt F, Degrave E. Occupational exposure to noise and the prevalence of hearing loss in a Belgian military population: A cross-sectional study. Noise Health [serial online] 2011 [cited 2019 Jul 16];13:64-70. Available from: http://www.noiseandhealth.org/text.asp?2011/13/50/64/73997

  Introduction Top


Excessive noise is considered as one of the most common occupational hazards. Based on the World Health Organization (WHO) definition, [1] about 16% of hearing loss (HL) worldwide is attributable to occupational noise exposure. The armed forces are among the occupational categories that are the most exposed to noise at work. [2],[3] The magnitude of HL that results from excessive exposure to noise depends on factors associated with the exposure (e.g., sound pressure level, duration, type of noise, and frequency), as well as the characteristics of the individual being exposed (e.g., susceptibility to noise damage, age, prior history of hearing/ear damage). Besides occupational exposures, HL has been associated with aging, [4],[5],[6] smoking, [7],[8],[9] leisure-time activities [10],[11],[12],[13] (rock concert, walkman, discotheque, etc.), and medical history [12],[13] (otitis media, head injuries, wearing a ventilation tube, etc.).

People serving in the military, especially those in combat units will at some point be exposed to high intensity noise. The sources of noise are of various types. They could be impulse noise from weapons systems, or continuous noise from jet engines, vehicles, communication systems and industrial-type activities. Unprotected exposure to impulse noise and continuous noise can result in a sensorineural hearing impairment called Noise Induced Hearing Loss (NIHL) that develops slowly over a long period of time and that begins at the highest frequencies (3-6 kHz). [14] Because its onset is slow, a person with NIHL may not be aware of the impairment at first. People with NIHL may experience several symptoms such as concomitant tinnitus, gradual decrease in sound, or a growing sound distortion, particularly regarding speech comprehension. Such HL can dramatically impact the person's operational capacity. NIHL is permanent and irreversible but it is preventable by the use of hearing protectors when exposed to noise.

It is therefore of interest to explore the association between military occupational noise exposure and the severity of HL in order to improve the efficiency of prevention strategies. To the authors' knowledge, there is no population-based epidemiological study to evaluate such an association in a military environment. The aim of this study was to characterize the severity and pattern of HL among the military personnel of the Belgian defence force and to evaluate its association with noise exposure.


  Methods Top


A cross-sectional study was carried out at the Centre for Medical Expertise (CME) located in Queen Astrid Military Hospital in Neder-Over-Hembeek, Belgium, and in four Units of Occupational Medicine (UOM) located in Marches-en-Famenne, Rocour, Leopoldsburg and Arlon, Belgium. The study protocol was approved by the bioethical committee of Queen Astrid Military Hospital. The population of interest was composed of all the military personnel serving in combat units (infantry, artillery, Paracommandos, recce and armored units), in the Navy, and in the Air Force [pilots, on-board crew, Air Defence Controllers (ADC) and Air Traffic Controllers (ATC)]. All personnel of the units of interest were routinely scheduled for hearing tests at one of the five locations, depending on the unit to which they belonged.

Eligible subjects were stratified by military category (infantry, artillery, Paracommandos, armored units, Navy and Air Force). The sample size was computed in each category according to an expected prevalence (obtained from a preliminary study) in order to achieve a precision of 5% and a type I error of 5% for the estimation of the prevalence of HL. All personnel of the units of interest, who were scheduled for a routine hearing test between January 1, 2006 and September 30, 2007, were invited to participate at the time of their clinic visit until the desired sample size was obtained. No restrictions were placed on age, gender, rank or length of military service. Of the 2725 contacted subjects, 2055 (75.4%) agreed to participate in the study.

Upon arrival at the CME or at the UOM, subjects were given the informed consent letter. Those who volunteered contributed their audiogram and were invited to complete a self-administered 48-item survey. The survey included questions about age, gender, rank, and unit, length of military service, work-related noise exposure history, use of hearing protection, medical history, smoking habits, and civilian noise exposure. Military noise exposure comprised number of years of riding in a tracked vehicle, time spent working in a noisy environment (e.g., in a vehicle maintenance installation) or with noisy tools, number of flights in a C130 airplane, number of weeks of military exercise in a year, frequency of small and large caliber shooting per year, number of Fighting in Built-Up Area (FIBUA) training sessions, yearly number of parachute jumps and number of years of navigation on a ship. Participants were asked about the following noisy leisure-time activities: using a personal stereo player (walkman, MP3, etc.), going to a discotheque or to a rock concert, car tuning with power stereo, using noisy tools (e.g., a chainsaw, etc.), driving a motorcycle, playing a musical instrument, attending or participating in motor sports, or flying in leisure aircraft.

Audiometric testing was performed individually in a sound-proof booth with an Interactoustics Clinical Audiometer (Model AC30) or a Damplex Automatic Screening Audiometer (Model AS54). Hearing thresholds (with an increment of 5 dB) were measured by trained medical personnel in the UOM or by audiologists in the CME once at each of seven pure tone frequencies (0.5, 1, 2, 3, 4, 6 and 8 kHz), for both ears. For the purpose of this study, HL was defined as thresholds ≥25 dB at 4 or 6 kHz in one or both ears according to the following classification: slight (25-40 dB), moderate (45-60 dB) and severe (>60 dB).

Demographic data about non-participants were obtained from the Belgian defence dataset. The non-participants were subjects from the same categories of personnel as participants but who were not in the study either because they had not been sampled or because they had refused to contribute their audiograms.

In each case, the subject's age was highly correlated with the number of years of service (Spearman correlation coefficient = 0.97, P < 0.001). Since age is a well-known risk factor for HL, age rather than number of years of service was included in all models as a confounding variable. Sex is also a well-known risk factor for HL but it was not included as a confounding variable because of the small number of women (5.2%) in our database.

The first goal of the analysis was to characterize the severity and pattern of HL in our population. A multinomial logistic regression age-adjusted model was used to assess the association of the severity of HL (none, slight, moderate or severe) with the presence of tinnitus and with belonging to a military occupation or Force.

The second goal of the analysis was to evaluate the association between the severity of HL and military noise exposures. Age (years of service) and exposure to different noise sources during civilian life were considered as potential confounders of the association. We implemented a step-forward procedure to select the variables in our data that were the most associated with HL, entering them in order of their P values, the smallest first. We used multinomial logistic regression to study the effect of noise exposure on three levels of severity of HL (slight, moderate and severe).

In order to take into account the effect of self-reported use of hearing protection, a categorical variable was created by making the product of frequency of exposure with the use of a hearing protection (never, sometimes or always). These new variables were created for military noise exposure variables and included in the final multinomial logistic regression.

Age was included as a continuous variable throughout the analysis. Dummy variables were used to code categorical variables. Exposure-response trends were estimated on the basis of incremental odds ratios over the categories and were also analyzed by entering categorical variables as continuous parameters in the models. A multinomial logistic regression model was chosen to assess the relationship between noise exposure and HL because of the categorical nature of the outcome. However, due to the large prevalence of HL, odds ratios have to be interpreted qualitatively and cannot be considered to approximate risk ratios.

All P values were two-tailed, and P values ≤0.05 were considered statistically significant. Odds ratios are presented with 95% confidence intervals (CI). All statistical analyses were performed using SPSS software (version 15.0; SPSS Inc., Chicago, IL, USA).


  Results Top


Overall, 2055 eligible subjects aged 18-55 years agreed to participate in the study. [Table 1] shows the characteristics of the participants and non-participants. The median age of the participants was 36 years and half the subjects had been employed for less than 17 years in the defence forces. Just over half the participants were in the enlisted category. More than a third of the participants were French-speaking. Most of the participants (77.4%) were from the Land Force. Among the participants of the Land Force, 29.4% of the subjects were from armored units. Eighteen (1.8%) participants from the Land Force were working in an Air Force function (as a pilot, ADC or ATC, as a mechanic or a steward). For the personnel of the Air Force, 39 (15.7%) participants were working as mechanic, gunner, loadmaster or steward. The 37 non-participants in the categories other than the Air Force were loadmasters or stewards since it was impossible to identify gunners inside the Air Force. The demographic characteristics were fairly evenly distributed among the two groups, although participants tended to include a slightly higher proportion of subjects from the Land Force compared with non-participants. Overall, the prevalence of HL was 55.8%. Of those with HL, 57.6% had slight HL, 24.4% had a moderate loss, and 18.0% had a severe loss.
Table 1: Characteristics of participants compared to non-participants


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Increased odds of slight, moderate and severe HL were found for people experiencing frequent or occasional tinnitus, compared to subjects who never experienced tinnitus [Table 2].
Table 2: Age-adjusted odds ratios for slight, moderate and severe hearing loss associated with the frequency of tinnitus


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Risks of slight, moderate and severe HL were significantly higher for subjects from the Land Force compared to subjects from the Air Force [Table 3]. More precisely, belonging to each trade of the Land Force was significantly associated with higher risks of slight, moderate and severe HL compared to subjects from the Air Force. Further investigations (data not shown) showed that subjects from Paracommando and infantry units were significantly more likely to have severe HL compared to subjects from the artillery (OR = 4.01, CI: 2.19-7.36 and OR = 3.03, CI: 1.68-5.47, respectively, for Paracommando and infantry versus artillery) and from the armored units (OR = 2.77, CI: 1.67-4.61 and OR = 2.10, CI: 1.28-3.42, respectively, for Paracommando and infantry versus armored units). Subjects from Paracommando units were also significantly more likely to have a moderate HL compared to subjects from the artillery (OR = 1.79, CI: 1.08-2.95) and from armored units (OR = 2.03, CI: 1.27-3.26). In a less pronounced way, subjects from the infantry were significantly more at risk of moderate HL compared to subjects from armored units (OR = 1.62, CI: 1.04-2.52).
Table 3: Age-adjusted odds ratios for slight, moderate and severe hearing loss associated with force and trade


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The multinomial logistic regression model obtained after step-forward selection included age, large caliber shooting, FIBUA training and participation in exercises, but excluded all civilian noise exposure [Table 4]. Not surprisingly, older age significantly increased the odds of slight, moderate and severe HL. Subjects who declared they participated in shooting with large caliber weapons 15 times or more a year were significantly more likely to have severe HL compared to subjects who declared they never shot with large caliber weapons, and subjects who declared they shot less than 15 times a year (OR = 2.40, CI: 1.34-4.20, not shown in [Table 4]). Increasing frequencies of FIBUA training and exercises were significantly associated with an increased risk of moderate and severe HL.
Table 4: Multiple-adjusted odds ratios for slight, moderate and severe hearing loss associated with military noise exposure


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After adjustment for age and participation in exercises, use of hearing protection modified the relationship between military noise exposure (large caliber shooting and FIBUA) and the risk of slight, moderate and severe HL [Table 5]. The risk of HL seemed to fall for subjects who declared they always used protection compared to subjects who declared they sometimes used protection, especially for those exposed 7 times or more to FIBUA training and those who participated 15 times or more a year in large caliber shooting. Quite surprisingly, it appears that subjects who declared they sometimes used protection were not protected against moderate and severe HL, and had even higher risks than those who declared they never used protection when exposed more than 7 times to FIBUA training.
Table 5: Multiple-adjusted odds ratios for slight, moderate and severe hearing loss associated with military noise exposure with or without protection


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Finally, a comparison of subjects without HL and subjects with severe HL showed that the latter were more likely to belong to Paracommando and infantry units of the Land Force, were more likely to be older and to have worked for a longer time in the Defence force, were more likely to participate in large caliber shooting more than 15 times a year (sometimes using hearing protection), were more likely to be exposed to FIBUA training (sometimes using hearing protection), and were more likely to participate in military exercises. Thus, HL is better explained by Land Force activity noise exposure.


  Discussion Top


HL was very common in our study population, affecting 55.8% of military personnel of 18-55 years of age. Differences between participants and non-participants suggest that our results may slightly overestimate the true population prevalence, as participants were slightly more likely than non-participants to belong to the Land Force, which was highly associated with HL.

The prevalence of slight, moderate and severe HL increased greatly with age. This pattern is consistent with other studies.[4],[5],[6],[7],[15],[16] However, many subjects in our database were younger than 55 years and should not have had an HL > 20 dB of 4 and 6 kHz according to the ISO norm 7029. [4] This increase of HL with age may reflect the number of years of military noise exposure at work, associated with the number of years of service.

Compared to subjects from the Air Force, subjects from the Land Force had a greater risk of moderate and severe HL, especially subjects from Paracommando and infantry units. Subjects from the Land Force may have a greater risk of HL due to greater noise exposure in occupational settings. This explanation is consistent with our findings regarding the association between military noise exposure and HL. Of the military noise exposure, FIBUA training, large caliber shooting and participation in exercises were the strongest determinants of moderate and severe HL. These activities are specific for the Land Force, particularly for the Paracommando and infantry units. The associations of FIBUA training and large caliber shooting with severe HL are consistent with the damaging effects of exposure to loud noise.

After adjustment for age and for other noise exposure variables, no significant association with leisure-time noise exposure was found in this cross-sectional analysis, except for a surprisingly protective effect of attendance at rock concerts and flying in leisure aircraft (data not shown). This protective effect was investigated to find a possible confounding effect among other factors. Introduction of the language variable in the model changed the P value for attendance at rock concerts and flying in leisure aircraft to nonsignificant. Language was not considered in our model because there is no biological explanation for this confounding factor to have an effect on the risk of HL.

After having introduced the self-reported use of hearing protection in the model, only exposure to large caliber shooting and FIBUA training while sometimes using protection were associated with HL (slight, moderate or severe). We failed to find a significant adverse effect of the absence of hearing protection when exposed to FIBUA or participating in large caliber shooting. This could be explained by the fact that very few subjects declared they never used protection during large caliber shooting or FIBUA training. We expected that subjects who declared they always used protection when exposed to FIBUA training or large caliber shooting would not be at increased risk of slight, moderate or severe HL. As the hearing protection equipment used by the Belgian defence forces is efficient, [17] we could explain this increased risk of HL despite hearing protections use by the fact that subjects may have changed their behavior toward protection following HL prevention campaigns. Another explanation of the absence of effect of hearing protections could be that subjects did not insert the earplugs deeply enough to benefit fully. A US study found that only 13.2% of the subjects inserted the earplugs deeply enough to achieve 22 dB noise attenuation in both ears. [18] This study also showed that the irregular use of hearing protectors did not protect enough against HL. This was confirmed by another study which found that the maximum exposure time decreases when intervals without protection are introduced. [19]

Comparisons of our prevalence findings with other published studies are difficult because of the lack of agreement on a standard definition of HL for use in epidemiologic studies, differences in age in the population tested, and differences in test frequencies. Prevalence studies in the military community are also scarce. Most of the studies have used the Significant Thresholds Shifts (STS), a difference between the baseline and annual audiograms, as a definition of HL to evaluate their hearing conservation program. In order to compare two populations, an age-adjusted comparison is necessary but we do not have information on the prevalence in each age category for the other populations.

The prevalence of HL in our study was compared with the prevalence found in three other published studies among the Finnish Defence Forces, Canadian military personnel and US Marine Troops. [15],[16],[20] In these studies, hearing was considered normal when the hearing threshold was 20 dB or less at all tested frequencies for the Finnish Defence Forces, at 4 and 6 KHz in the left ear for Canadian Forces and at 4 kHz in the left ear for US Marine Troops. Applying these definitions to our study population, the prevalence of HL appears to be slightly higher in the Finnish Defence Forces (67.9% of the officers in the Finnish Defence Forces versus 65.7% of the officers in our population) but lower in the Canadian Forces (34.4% on 4 kHz and 45.1% on 6 kHz versus 42.5 and 46.5%, respectively, in our population) and the US Marine Troops (10.8% of the subjects from artillery, infantry, special troops, ATC and armored units versus 33.7% in the same units of our population). These differences in prevalence of HL should be balanced by the fact that the populations were not equal in age. The population in the study of the Finnish Defence Forces and the Canadian Forces were older than the subjects from our population.

The main limitation of our study is its reliance on self-reported information over exposure to noise in a retrospective way because no objective measures were possible. A number of steps were taken to reduce or measure potential recall bias, including encouraging truthfulness through anonymity, helping subjects to recall frequencies by indirect questions, and comparing answers between subjects from the same units and with the same function. Another limitation of the present study is related to its cross-sectional design, implying that neither causal inferences nor a temporal relationship between exposure and HL can be drawn from our findings. Another limitation of a cross-sectional design might be the "healthy soldier effect". That is, healthy soldiers remain employed, while unhealthy soldiers with HL tend to lose medical fitness and are no longer present to be included in cross-sectional studies. However, the "healthy soldier effect" is probably not a problem in our study because audition criteria to stay medically fit in the Belgian army are based on frequencies 500, 1000 and 2000 Hz, whereas we considered the frequencies of 4000 and 6000 Hz to define HL in our study. Even if losses tend to spread to the lower frequencies with continued noise exposure, this does not lead to loss of medical fitness. There is no healthy soldier effect here as military personnel with HL on 4000 and 6000 Hz were included in the study.

Another possible limitation of our study is that the medical personnel who performed the audiometric testing, although trained, were in general not professional audiologists. As an audiometric test always has a somewhat subjective element, the results may be slightly inaccurate because trained medical personnel could be misled by the subject examined. Their measurements may therefore be subject to bias, and the direction of this bias may depend on what the subjects want to achieve. The only subjects who might have had an interest in influencing the results of the test in our study were from Paracommando units, the Air Force or the Navy Force because they want to stay medically fit (they might pretend to hear something even if they cannot). However, we can assume that the bias due to hearing examination was very slight in our study because the hearing of these subjects was tested by audiologists.

The main strength of the current study is the availability of information on different sources of noise to which military personnel are exposed. Previous prevalence studies of HL among military personnel in the literature did not collect[11],[15],[20] or use [16] information of exposure to noise sources in a military environment. This could be explained by the fact that exposure to noise in a military environment is very difficult to characterize, as military personnel are exposed to various types of noise sources (from impulse to continuous noise) at various frequencies and for different lengths of time. Exposure to noise in a factory is much easier to quantify because workers from this type of sector are exposed to the same sources of noise every work day and during the whole work day, so that researchers can calculate a noise dosage [5] (a combination of the exposure time and the noise level).

The high prevalence of HL in our military population indicates that HL is an important health problem in the Belgian defence force. These results suggest that subjects from infantry and Paracommando units are at the most risk of HL because they are exposed to very loud noises in their professional life, such as shooting large caliber weapons and FIBUA training. Therefore, we suggest the use of nonlinear earplugs [21] or active noise reduction headsets when exposed to impulse noise. We also underline the importance of efficient training to guarantee that soldiers insert the earplugs deeply enough [18] and wear hearing protectors for 100% of the time in a noisy environment. [19] A prospective study is needed to further investigate the impact of military noise exposure and the use of hearing protectors on HL.


  Acknowledgment Top


We thank Mr. Willy Lienard and Ms. Jeannine De Leeuw from the Unit of Epidemiology and Biostatistics of the Staff Department Well Being for their valuable assistance in realizing this study.

 
  References Top

1.World Health Organisation. Reducing risks, promoting healthy life. World Health Report. Chapter 4 Quantifying selected major risks to health; Paragraph 9 Selected occupational risks. Work related noise; 2002. p. 76-7.   Back to cited text no. 1
    
2.National Institute for Occupational Safety and Health. Revised criteria for a recommended standard - Occupational Noise Exposure, US Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOH) Publication; 1998. p. 98-126.  Back to cited text no. 2
    
3.Paoli P, Merllié D. Third European Survey on Working conditions 2000. European Foundation for the improvement of living and working conditions.  Back to cited text no. 3
    
4.ISO 7029: Acoustics-Statistical distribution of hearing thresholds as a function of age. Geneva: International Organization for Standardization;2000.  Back to cited text no. 4
    
5.Bauer P, Körpert K, Neuberger M, Raber A, Schwertz F. Risk factors for hearing loss at different frequencies in a population of 47,388 noise-exposed workers. J Acoust Soc Am 1991;90:3086-98.  Back to cited text no. 5
    
6.Rosenhall U. The influence of ageing on noise-induced hearing loss. Noise Health 2003;5:47-53.  Back to cited text no. 6
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8.Cruickshanks KJ, Klein R, Klein BE, Wiley TL, Nondahl DM, Tweed TS. Cigarette smoking and hearing loss: the epidemiology of hearing loss study. JAMA 1998;279:1715-9.  Back to cited text no. 8
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9.Mizoue T, Miyamoto T, Shimizu T. Combined effect of smoking and occupational exposure to noise on hearing loss in steel factory workers. Occup Environ Med 2003;60:56-9.  Back to cited text no. 9
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10.Zenner HP, Struwe V, Schuschke V, Spreng M, Stange G, Plath P, et al. Hearing loss caused by leisure noise. HNO 1999;47:236-48.  Back to cited text no. 10
    
11.Jokitulppo J, Toivonen M, Björk E. Estimated leisure-time noise exposure, hearing thresholds, and hearing symptoms of Finnish Conscripts. Mil Med 2005;171:112-6.  Back to cited text no. 11
    
12.Job A, Raynal M, Rondet P. Hearing loss and use of personal stereos in young adults with antecedents of otitis media. Lancet 1999;353:35.  Back to cited text no. 12
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13.Job A, Raynal M, Tricoire A, Signoret J, Rondet P. Hearing status of French youth aged from 18 to 24 years in 1997: a cross-sectional epidemiological study in the selection centres of the army in Vincennes and Lyon. Rev Epidemiol Sante Publ 2000;48:227-37.  Back to cited text no. 13
    
14.Clark W, Bohne B. Effects of Noise on Hearing. JAMA 1999;281:1658-9.  Back to cited text no. 14
    
15.Ylikoski M. Self-reported elevated blood pressure in army officers with hearing loss and gunfire noise exposure. Mil Med 1995;160:388-90.  Back to cited text no. 15
    
16.Abel S. Hearing loss in military aviation and other trades: Investigation of prevalence and risk factors. Aviat Space Environ Med 2005;76:1128-35.  Back to cited text no. 16
    
17.Bockstael A, Keppler H, Dhooge I, D′Haenens W, Maes L, Philips B, et al. Effectiveness of hearing protector devices in impulse noise verified with transiently evoked and distorsion product otoacoustic emissions. Int J Audiol, 2008;47:119-33.   Back to cited text no. 17
    
18.Bjorn V, Albery C, McKinley RUS. Navy Flight deck hearing protection use trends: survey results. In New Directions for Improving Audio Effectivness. In New Directions for Improving Audio Effectivness (pp. 1-1 - 1-20). Meeting Proceedings RTO-MP-HFM-123, Paper 1. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.aps.   Back to cited text no. 18
    
19.Van Wijngaarden S, Soo J. Protecting Crew Members against Military Vehicle noise. Paper presented at the NATO Research and Technology Organization Applied Vehicle Technology Symposium on "Habitability of Combat and Transport Vehicles: Noise, Vibration and Motion", held in Prague, Czech Republic; 2004. p. 4-7.  Back to cited text no. 19
    
20.Barney R, Bohnker B. Hearing thresholds for U.S. Marines: Comparison of aviation, combat arms, and other personnel. Aviat Space Environ Med 2006;77:53-6.  Back to cited text no. 20
    
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Correspondence Address:
Audrey Collee
Unit of Epidemiology and Biostatistics, Staff Department Well Being, Bruynstreet 1, 1120 Brussel
Belgium
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1463-1741.73997

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]

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