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|Year : 2011
: 13 | Issue : 53 | Page
|The influence of military service on auditory health and the efficacy of a hearing conservation program
Per Muhr1, Ulf Rosenhall2
1 Department of Clinical Neurosciences, Clinical Science, Intervention and Technology, Division of ENT and Hearing, Karolinska Institute; The Swedish Armed Forces, The Occupational Health Centre at the Garrison of Halmstad, Sweden
2 Department of Clinical Neurosciences, Clinical Science, Intervention and Technology, Division of ENT and Hearing, Karolinska Institute; Department of Audiology, Karolinska University Hospital, Stockholm, Sweden
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|Date of Web Publication||14-Jul-2011|
The influence of military service on self-assessed hearing symptoms and measured auditory function was studied as well as the efficacy of the Hearing Conservation Program (HCP) of the Swedish Armed Forces. 839 conscripts were recruited for the study at reporting to military service. They were all exposed to noise over the risk-limits from weapons and vehicles and used earmuffs and/or earplugs. Questionnaires and pure tone screening audiometry were studied at the start and the end of the military service. Retrospective information regarding audiometry at conscription before military service was included as control. The prevalence values of tinnitus were 23% before and 32% after the service and of sensitivity to noise 16% and 19% respectively. The prevalence values of hearing impairment were 6.3% at conscription, 14.5% at reporting to military service, and 24% after the training period. The incidence values of hearing decline were 3.7% during the period with no military noise exposure and 6.6% during the military service. Acoustic accident increased the risk of worsened tinnitus and sensitivity to noise four times and for a high frequency hearing decline six times. We observed elevated prevalence values of tinnitus, sensitivity to noise and hearing impairment at discharge compared to before military service. We observed an elevated risk of hearing decline during military service. Acoustic accident increased the risk of tinnitus, noise sensitivity and hearing decline. We suggest improvements regarding inclusion criteria for military service, and for education regarding the HCP.
Keywords: Conscript, hearing impairment, hearing conservation program, noise, tinnitus
|How to cite this article:|
Muhr P, Rosenhall U. The influence of military service on auditory health and the efficacy of a hearing conservation program. Noise Health 2011;13:320-7
| Introduction|| |
In military service a variety of intense noise exposures implies pronounced hazards for the auditory health of the conscripts. Noise-induced hearing loss (NIHL) and noise-induced tinnitus has been reported to occur frequently during military service. ,,[ 3] In an earlier study with similar design but involving another military study group we observed an increase in the incidence of hearing decline during the service period.  Military service is performed in young adult life, during which a multitude of other factors also might result in hearing impairment (HI) and tinnitus. Such factors are leisure time and occupational noise exposure, middle ear disease, genetic factors, and head trauma. ,,
A hearing conservation program (HCP) is of the utmost importance to prevent auditory complications during military service. The HCP of the Swedish Armed Forces (SAF) includes sound level measurements, noise control, safety instructions and education for all officers and soldiers, the use of hearing protection devices (HPD) and repeated audiometric testing. The compliance of an HCP is dependent on knowledge and attitude among the conscripts.  All conscripts are educated in the risks and consequences of NIHL, levels of noise exposure from weapons and vehicles and how to protect themselves in accordance with the Safety Regulations. The soldiers used earmuffs, helmets with earmuffs, or a combination of earplugs and earmuffs. The earmuffs used were equipped with electronic level-dependent function to allow speech communication.
In the 1990s large numbers of conscripts reported auditory complications, and the number of conscripts who suffered from NIHL during military service was relatively large.  For this reason a revised, stricter military HCP was introduced in 2001-02, and this HCP is currently in force.
The aims of the study were to investigate the aural health of conscripts drafted for military service regarding:
- The influence of military service on self-assessed hearing symptoms and measured auditory function
- Prognostic considerations regarding tinnitus and other ototraumatic indicators on auditory deterioration during military service
- Knowledge, attitude to, and the efficacy of the current HCP in the Swedish Armed Forces
The participants were recruited from young Swedish male conscripts reporting to primary military service in the SAF. The collection of data started in June 2002 and ended in April 2005. All conscripts had to pass a medical and psychological examination at conscription at the age of 18 years. The audiometric inclusion criterion to be drafted for military training at the time of the study was pure tone thresholds ≤25 dB HL for the frequencies 0.5 to 8 kHz in both ears. Pure tone thresholds ≤30 dB HL at one or more frequencies, and 35-40 dB HL at one single frequency, could be accepted after consultation with an ENT specialist. Accordingly, only men with normal hearing or at most slight hearing impairment (HI) were included in this study. In an earlier study we observed a prevalence value of HI of 14.3% (at 3, 4 or 6 kHz in right or left ear) in an unselected group tested in 1995, before the audiometric inclusion criteria had been applied. 
A detailed description of the study group is presented in an earlier report.  The total number of conscripts included in the study was 839, all men. Most of the participants (n: 702) were servicemen who reported to an anti-aircraft unit in 2002. Another group of 100 participants was enrolled from an artillery regiment, gun crews at the regiment in 2002-03. A third group of 40 servicemen was recruited from an armored regiment in 2004. They were enrolled as crewmembers of armored vehicles. When reporting to military service, 78% of the participants were 19 years of age, 21% 20 years, and 1% 21 or 22 years (median age 19 years). The duration of military service for the participants who completed the training was on an average 243 days (eight months), varying from 223 to 342 days.
The servicemen in this study were exposed to noise from weapons and vehicles. During a basic training period of six weeks all soldiers were exposed to 100-300 shots of live ammunition from the own rifle, some hand grenade explosions and some shots from a light anti-tank weapon. None of these servicemen from the anti-aircraft unit were exposed to noise from anti-aircraft guns. The artillery crews were exposed to impulse noise from small-caliber weapons in their basic training, but after that mainly to intense low-frequency impulse noise from large-calibere weapons (several hundreds of shots from howitzers). The servicemen from the armored regiment were mainly exposed to high levels of low-frequency stationary noise from the armored vehicle during approximately 300-350 h. They were also exposed to impulse noise exposure from small- and large-caliber weapons. Measured sound levels are presented in [Table 1] (unpublished data, classified reports, The Army Safety Inspectorate). All soldiers in this study used the mandatory earmuffs, and/or earplugs stated in the safety instructions.
| Methods|| |
The study was designed as a longitudinal prospective study, covering the training period from reporting to military service to discharge from it. The study included enquiries and pure-tone screening audiometry from 10 dB HL. This was performed at 1) reporting to training, and 2) at the end of the military service. In case of a premature discharge from military service the second questionnaire was filled in and audiometry was performed. Retrospective information regarding pure-tone screening audiometry of the study group at conscription before military service was included. The incidence of hearing decline in the study group in the period from conscription to reporting to service (mean 13 months), was used as a control of the incidence of hearing decline during the military training period.
A more detailed description of the design and validity of part of the questionnaire is presented in an earlier report.  Since there was no appropriate validated questionnaire available in Swedish we designed a questionnaire suitable for the purpose of the study. Three of the questions were adopted from a validated questionnaire  and the practical applicability of the questionnaire was tested on 20 conscripts. Questionnaire #1 was filled in at the first part of the medical examination at reporting for duty.  The questionnaire had three sections, and two of them were used in this study. One section contained items on auditory symptoms like tinnitus and sensitivity to noise ([Table 2], item #1-4, 13). The second section contained items on noise exposure (item #7-12). Questionnaire #2 was distributed at the end of the training period and the time period asked for was from reporting to training to discharge from military service. The questions of Section one and two in questionnaire #1 were used in questionnaire #2 at discharge, as well as two new sections of questions. These sections focused on the education performed as part of the Army HCP ([Table 2], item #19-22), the experience of failure of protection, and problems to use the HPDs during the training period (item #23-26). The response alternatives in item #1-4, #7-12 and #23-26 were "No, not at all", "Yes, sometimes/a few times" and "Yes, often or always/many times". An increase of a symptom is thus measured in terms of occasional or frequent occurrence. Item #13 there was only two response alternatives, "Yes" and "No". In item #19-22 the response alternatives were "Yes, but too detailed", "Yes, adequate" and "No, insufficient". The English translations of the items are included in [Table 2].
The response rate for the questionnaires was 100% at reporting for training and 94.6% at the end of the service. The response rate for the single items was 96.8-99.8%.
All audiometric tests were mandatory. They were performed at the Conscription Centers and at the Army Health Units in soundproof booths. Entomed SA 250 computer-based audiometers with Sennheiser HDA 200 earphones were used. The audiometers were calibrated once a year according to an edition preceeding ISO 389-8, 2004  and fulfilled the standard IEC 60645-1, 2001.  Air conduction thresholds were measured according to the shortened ascending method.  The participants were screened from the level 10 dB HL at the frequencies 0.5, 1, 2, 3, 4, 6 and 8 kHz in both ears separately. The thresholds above the screening level were measured and recorded in 5-dB steps. The audiometric results are presented as prevalence values of hearing impairment (HI) with thresholds above 20 dB HL in one or both ears and at one or more frequencies in the interval 0.5-8 kHz. Means and medians could not be calculated since thresholds below the screening level 10 dB HL were not measured. Noise-induced hearing loss is predominantly found in the frequency range 3, 4 and 6 kHz,  and HI above 20 dB HL affecting one or both ears at one or more of these frequencies is reported as high-frequency HI. Hearing impairments involving the frequencies 0.5, 1, and 2 kHz are reported as low- and mid-frequency HI. A significant threshold shift (STS) for a single subject was defined as a decline in thresholds between two audiometric tests of 15 dB or more in at least one ear and one tested frequency. A STS had to be verified in a second test at least one day later to be recorded.  The incidence values of STS were defined as the proportion of subjects in the study group with a STS during the period from conscription to reporting for training or from reporting to training to discharge.
The PC software SPSS 14.0 was used for data storage and the statistical procedures. The Chi-square test was used for the statistical tests in the questionnaire and for prevalence values of HI. A multinomial logistic regression analysis was performed at discharge. Cumulative incidence values of hearing decline (STS) before and during military service are presented. The relative risk (RR) was calculated as the ratio between the incidence values during military service and the period before the service with no military noise exposure. In the statistical tests 0.05 was applied as level of significance.
The Ethics Committee at Karolinska Institutet, Stockholm, Sweden approved this study.
| Results|| |
Auditory symptoms before and after the training period
Self-assessed auditory symptoms had a tendency to increase during the training period [Table 2]. Tinnitus (item #3) had an overall prevalence of 23% at the start and of 32% at the end of military service and sensitivity to noise (item #4) increased from 16 to 19%.
Incidence values of auditory symptoms
The overall incidence of tinnitus (increasing one or two steps, item #3), during the training period was 17%. Some of the conscripts (6%) reported less tinnitus symptoms after the military service compared to before the training. The incidence of sensitivity to noise (increasing one or two steps, item #4), during military service was 12%, and 7.1% reported less sensitivity to noise. Twelve percent of the conscripts reported hearing deterioration (item #13) before military service, and 26% during the training period.
Hearing impairment and hearing decline
The prevalence values of HI for all measured frequencies (0.5-8 kHz) were 6.3% at conscription, 14.5% at reporting to military service, and 24% at the end of the period. These differences were statistically significant for the frequency intervals described in [Table 3]. The auditory thresholds varied within the limits 10-30 dB HL at conscription, 10-40 dB HL at reporting and 10-50 dB HL at discharge.
The cumulative incidence values of STS for all frequencies 0.5-8 kHz were 3.7% during the period with no military noise exposure and 6.6% during the military service period
[Table 3]. The relative risk for a STS during the military training was 1.8 (RR) at 0.5-8 kHz (95% CI, 1.2-2.7) and 2.4 (RR) at 3-6 kHz (95% CI, 1.2-5.0). At 0.5-2 kHz and at 8 kHz we did not observe any significant effects.
We observed a tendency of increased risk of STS during the training period among those conscripts who had mild HI at reporting compared to those with normal hearing at reporting (OR=1.8, P=0.084). Those conscripts who reported tinnitus or noise sensitivity at reporting to training had about the same incidence of STS during the training period as those of the contrast group.
Leisure time and accidental noise exposure
The prevalence values of leisure time noise exposure decreased during the military training period compared to before it ([Table 2], item #7-10). Before the military service 44% of the study group attended rock concerts or discotheques often or many times compared to 21% during the service period. The corresponding figures for playing loud music in a band was 17% and 6.1% respectively. Also, for shooting or hunting and motor sports the leisure time noise exposure decreased during the service period.
Temporary tinnitus after exposure to loud noise was common, both during the military service and before ([Table 2], item #11). Such incidents occurring often/many times were reported more frequently before the military service (9.4%) than during the training period (2.7%). About two-thirds of the servicemen reported exposure to loud noise without wearing HPDs ([Table 2], item # 12). Frequent (often/many times) exposure to excessive noise without HPDs was more prevalent before (12%) than during the training period (5.4%).
A logistic regression analysis was performed at discharge with items #3 (tinnitus), and #4 (noise sensitivity), and prevalence and STS at 3-6 kHz as dependent variables, and items #7-10 (leisure time noise), item #11 (tinnitus after loud noise), and #12 (loud noise without HPDs) as independent variables. Only item #11, tinnitus after loud noise exposure had a significant correlation to elevated prevalence values of tinnitus and sensitivity to noise at discharge (P<0.001).
Acoustic accidents caused by impulse noise exposure during the training period was reported to the officer in charge by 6.1% (n=51) of the conscripts. Such incidents enhanced the probability of increasing tinnitus and sensitivity to noise at discharge four times [Table 4]. The relative risk of a STS at 3-6 kHz at discharge for those conscripts who reported an acoustic accident compared to those who did not was 5.7 (95% CI, 2.2 - 15.1).
Interrupted training because of hearing problems
Some conscripts, 9.4% of the entire group, interrupted their service prematurely; the mean period from reporting to discharge in this group was 53 days. Hearing problems was the reason for interruption in 33 cases (3.9% of the entire group). For five servicemen an acoustic trauma was the reason, but for the others no special reason was given in the files. As can be seen in [Table 5] this group of 33 servicemen deviated significantly from the majority already at reporting to service regarding seven of the items.
The Hearing conservation program
A majority of the conscripts found the education adequate ([Table 2], item #19-22). However, 20% of the respondents found it insufficient concerning information about risk factors for hearing loss. A majority found the education adequate, or even too detailed, regarding the NIHL risks from the weapons used and the use and the maintenance of hearing protectors. Half of the study group found the education concerning how to act in case of acute HI insufficient.
The statement that the HPDs often did not provide adequate protection was shared by 3.9% of the servicemen ([Table 2], item #23). More than half of the servicemen reported HPDs with bad fitting and sound leakage occasionally during the training period (item #24). The experience that this occurred often/many times was expressed by 5.6% of the participants. One-fifth of the study group found the HPDs uncomfortable to wear often or many times (item #25). A vast majority of the servicemen (93%) reported that the level-dependent function of the hearing protectors made conversation possible most of the time (item #26). The conscripts who had tinnitus at reporting for training expressed similar opinions about and attitudes towards HPDs as the contrast group. This was also the case for the conscripts who reported sensitivity to noise at reporting, with one exception: They found the attenuation of the HPDs less sufficient compared to the contrast group (P<0.001).
| Discussion|| |
There are some limitations in the screening method for estimating the prevalence of HI. The shortened ascending method may be less accurate compared to clinical audiometry but studies have shown no significant differences in threshold values and standard deviations between the methods.  In a study from 2007 we validated the screening method used here against clinical audiometry at the frequencies 3, 4 and 6 kHz and concluded, "the comparison indicated a high degree of reliability of the screening procedure".  Screening from 10 dB HL conceals thresholds below 10 dB HL. The consequence is that means and medians cannot be calculated and a relevant method for presenting the results is to use prevalence values of HI.
Standard deviations of the test-retest difference scores for the method used are 3-7 dB depending on frequency. Some degree of STS is expected only by chance. The theoretical calculated probability to have at least one positive STS of 15 dB or more at any frequency and ear between two measurements only by random given a SD of 5 dB is 3.5%. ,
This figure is very close to the STS in our comparison group that was confirmed by a retest (3.7%). Deterioration, for example, from 0 to 15 dB cannot be detected by using screening from 10 dB. The results could be biased with too low estimated incidence values of STS in the comparison group. However, the method used was the same throughout the investigation for both the study group and the comparison group and this allows a comparison between them. The dropout frequency is low throughout the investigation minimizing the error.
Exposure to noise
The conscripts were frequently exposed to impulse noise and intense continuous noise exceeding the risk limits during the service. HPDs were mandatory in all noisy situations. In a recent Finnish study  it was estimated that during military service 89% of the conscripts were exposed over the L pAeq,8 85 dB risk limit. The exposures from leisure time noise in our study group decreased considerably during the training period to about half as frequent compared to before. This is in accordance with the results in a Finnish study. 
The education regarding risk factors for hearing loss was estimated as adequate by 80% participants of this study, and 95% of them reported that they had adequate knowledge about risk areas around weapons and the use of HPDs. In a British study  inadequate education of the HCPs was reported, and all the infantry soldiers in this study had experienced temporary hearing loss. Moreover, communication difficulty was the most acknowledged problem. The HPDs used in this study had no level-dependent function, in contrast to the present study in which a vast majority of the conscripts reported that the level-dependent function of the HPDs made conversation possible most of the time. We observed a decreasing prevalence of frequent exposure to excessive noise without HPDs during the training period compared to before the service, probably an effect of the education and the mandatory use of HPDs. However, one-fifth of the conscripts often found the HPDs uncomfortable to wear and 4-6% experienced that they often leaked sound or did not provide adequate protection.
Auditory symptoms and hearing impairment
Large proportions of the young men in this selected study group reported hearing symptoms at the start of the military service.  The prevalence values of most auditory symptoms asked for increased from reporting to military service to discharge. Jokitulppo et al., reported the opposite situation with decreasing prevalence values of tinnitus and sensitivity to noise. In our material the participation rate was high both at reporting to service and at discharge. In the Finnish study the participation rate was high at reporting to service but low at discharge (42%). These self-assessed results could be biased from changes in attitudes between reporting and discharge among the conscripts. We assume the influence of recall bias to be minimized according to the wording of the questions.
We observed increasing prevalence values of HI from reporting to service to discharge [Table 3]. This is in accordance with the findings by Jokitulppo et al., The slightly lower prevalence values in our study, compared to the Finnish one, could be due to the strict hearing inclusion criterion that has been used in Sweden since some years. The increase in prevalence values of HI during the military service seems to be the same in Finland and Sweden, 8-9%. The observed higher prevalence values at 3-6, and 8 kHz, are in accordance with the results by Mrena et al., in the Finnish Defense Forces of officers exposed to impulse noise for many years, who had significantly elevated hearing thresholds at 4-8 kHz. In a Norwegian study  the effects of impulse noise was about the same from 4 to 8 kHz for men aged 20-44 years. Somma et al., reported HI of the high frequencies over 6 kHz, including 8 kHz, in cement workers exposed to stationary noise. These observations emphasize 8 kHz as a possible NIHL frequency, possibly related to impulse noise exposure.
The cumulative incidence values of hearing decline (STS) were larger during the military service than during the period from conscription to reporting for training [Table 3]. This difference is still larger, since the incidence values are cumulative and the first time period from conscription to reporting is longer than the second one, the military service. We observed a decrease in leisure time noise exposure during military service. Both the shorter time period in military service and the decrease in leisure time noise exposure during this period emphasize military noise as a risk factor. In an earlier study among Swedish conscripts we observed elevated incidence values of STS during military service compared to before.  The relative risk for an STS in the present study group with a stricter enforced HCP including hearing inclusion criteria was 1.8 compared to 2.7 in our earlier study but the confidence intervals of the relative risk values were overlapping. To our knowledge there are no other scientific studies on incidence values of hearing decline of conscripts during military service.
Predictive value of auditory symptoms before military service
Four percent of the servicemen interrupted the service prematurely because of hearing problems. Already at reporting to military service this group had more self-assessed hearing symptoms and HI compared to the others. A large proportion of the group had elevated hearing thresholds between conscription and reporting to service. Those conscripts who had multiple auditory symptoms and decline in hearing thresholds between conscription and reporting to service could represent a vulnerable group. In an earlier study,  when a less strict inclusion criterion was applied, an increased risk (RR=3.1) for a hearing decline was found among soldiers with HI already at reporting to training. A similar observation among conscripts was reported by Klockhoff et al. In the present study, with stricter drafting inclusion criteria, we could still observe a tendency of an elevated risk for hearing decline (RR=1.8, P = P.084) among the soldiers with HI already at reporting to training. Tinnitus and other aural symptoms present at reporting to service did not predict an increased risk for hearing decline. However, aural symptoms and HI prior to military service are still risk factors to be taken seriously, since they predict an increased probability of interruption of military service or hearing decline.
The hearing conservation program
We observed a tendency of decrease in hearing decline (STS) from 7.9% in the late 1990s  to 6.6% in the present study. The decrease in relative risk was from earlier 2.7 to 1.8 in the present study, but the confidence intervals were overlapping [Table 3]. These observations indicate a possible positive influence of the HCP in the SAF and it seems that the HCP reduces the risk of occupational NIHL during military service. However, the less restricted hearing inclusion criterion may be responsible for a higher prevalence of HI at conscription in the first study. This could inflate the estimated incidence values of STS at reporting to training and at discharge making the RR values in the two studies difficult to compare. The situation regarding hearing protection during military service is not yet satisfactory and exposure to military noise still imposes a risk for the servicemen. Aural symptoms, HI and STS were significantly more prevalent at discharge than at reporting to military service.
Acoustic accidents were reported to the officer in charge by 6.1% of the conscripts in our study. This group, most commonly exposed to impulse noise without HPDs, had a four to six times elevated relative risk of deteriorated tinnitus, noise sensitivity and STS, compared to the contrast group [Table 4]. Jokitulppo  reported that exposure to military explosions correlated to tinnitus. Mrena et al., studied effects of the enforcement of stricter safety regulations in the Finnish Defense Forces. Before, up to 15% of the Finnish conscripts suffered from HI associated with an acoustic trauma, and thereafter the figure was about 2%. Another indicator of high noise exposure and effect on the auditory system in our study is item #11, tinnitus after noise exposure. In the multinomial regression analyses we observed a significant correlation between item #11 and elevated prevalence of tinnitus and noise sensitivity at discharge. In an earlier study of young men before military service we also observed significant relations between item #11 and elevated values of tinnitus, noise sensitivity and HI. 
The results of the present study supply suggestions for improvements of the present military HCP. Three areas of improvement can be discerned:
1) Stricter selection criteria at enlistment. A short questionnaire, added to the conscription examination, provides information of aural symptoms. A combination of aural symptoms and slight HI could be recognized as a reason for exemption from service in the Armed Forces.
2) Education in HCP. The education regarding military safety regulations seems to be adequate. The education regarding the consequences of excessive exposure to noise seems to be inadequate. An even more active involvement of military medical personnel in the HCP can be recommended.
3) Use of HPDs. There are still problems regarding the use of hearing protectors among the servicemen. Poor fitting, inconveniences and acoustic accidents are commonly reported. A revision of the types of HPDs purchased and the education in the use of HPDs is recommended.
| Conclusions|| |
In the study group (with normal or nearly normal hearing) followed before and after the primary military training we observed:
- Elevated prevalence values of tinnitus, sensitivity to noise and HI at discharge compared to at reporting to military service
- Elevated incidence values of hearing decline (STS) during military service
- Elevated prevalence values of tinnitus and noise sensitivity at discharge among the conscripts who often had been exposed to "loud noise and thereby felt whistling or ringing in the ears"
- Elevated risk for increase in tinnitus, noise sensitivity and STS at discharge among the subgroup of conscripts who reported an acoustic accident
- Decrease in leisure time noise exposure during military service
- Auditory symptoms and HI prior to military service predict an increased probability of interruption of military service.
We suggest improvements regarding the inclusion criteria for service in the Armed Forces, and for education regarding HCP and HPDs.
| Acknowledgment|| |
The authors are grateful for the grants received from the Foundation Tysta Skolan and The Foundation for Audiological Research (The Stinger Foundation).
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Försvarshälsan, Lv 6, Box 515, 30180 Halmstad
Source of Support: Foundation Tysta Skolan and The Foundation for Audiological Research (The Stinger Foundation), Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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