Background Military aviators are potentially at risk for developing noise-induced hearing loss. Whether ambient aircraft noise exposure causes hearing deficit beyond the changes attributed to natural ageing is debated. The aim of this research was to assess changes in hearing thresholds of Israeli Air Force (IAF) pilots over 20 years of military service and identify potential risk factors for hearing loss.
Methods A retrospective cohort analysis was conducted of pure-tone air conduction audiograms of pilots, from their recruitment at 18 years of age until the last documented medical check-up. Mean hearing thresholds were analysed in relation to age, total flight hours and aircraft platform. Comparisons were made to the hearing thresholds of air traffic controllers (ATCs) who were not exposed to the noise generated by aircraft while on duty.
Results One hundred and sixty-three pilots were included, with flying platforms ranging from fighter jets (n=54), combat helicopters (n=27), transport helicopters (n=52) and transport aircraft (n=30). These were compared with the results from 17 ATCs. A marked notch in the frequency range of 4–6 kHz was demonstrated in the mean audiograms of all platforms pilots, progressing with ageing. Hearing threshold shifts in relation to measurements at recruitment were first noted at the age of 30 years, particularly at 4 kHz (mean shift of 2.97 dB, p=0.001). There was no statistical association between flying variables and hearing thresholds adjusted for age by logistic regression analysis.
Conclusions The audiometric profile of IAF pilots has a pattern compatible with noise exposure, as reflected by characteristic noise notch. However, no flight variable was associated with deterioration of hearing thresholds, and no significant difference from non-flying controls (ATCs) was seen.
- noise-induced hearing loss
- noise notch
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Military pilots are routinely exposed to ambient aircraft noise, which can potentially damage hearing acuity.
Slight hearing threshold shifts in the frequencies most susceptible to noise damage (3–6 kHz) were identified in audiograms of relatively young Israeli Air Force (IAF) pilots.
Hearing thresholds of pilots and an age-matched group of air traffic controllers (ATCs) who were unexposed to aircraft noise did not differ significantly.
Hearing protection by means of earplugs under headsets or helmets with active noise reduction systems has become standard for pilots in the IAF.
We believe that the use of earplugs is most likely the reason for comparable hearing thresholds of pilots and ATCs.
Noise-induced hearing loss (NIHL) remains a significant concern for employers and constitutes a major public health issue worldwide.1 It is characterised by impaired hearing acuity, particularly in the frequency range of 3–6 kHz, due to irreversible damage to sensory hair cells in the inner ear.2 This degenerative process can evolve as a result of exposure to continuous loud noise or intermittent high-level noise. Military aircrew personnel encounter excessive noise while on duty. Noise levels generated by aircraft vary according to phase of flight as well as environmental factors and can reach as high as 90–140 dB along the flight.3–5 At take-off, engines of most aircraft generate high-intensity noise, even for brief periods.4 Whether ambient aircraft noise exposure has deleterious effect on hearing acuity of pilots, beyond the expected age-related hearing loss, is debated. Type of aircraft, hours of flight and years of military service have been implicated as risk factors for the development of NIHL in aviators.3 6–8 Various studies have claimed that pilots are at increased risk for hearing loss5–7, whereas others report significant correlation of hearing threshold changes to age, thus identification of flying occupation as a cause for hearing loss is inconclusive.3 9 Some studies even suggested that air force personnel have better hearing in reference to the general population of equivalent age or no worse than predicted for estimated occupationally related NIHL.4 8 10
Israeli military aviators and air traffic controllers (ATCs) go through annual medical check-ups exclusively at the Israeli Air Force Aeromedical Center (IAFAMC), starting when recruited at 18 years of age and then yearly, as long as maintaining active reserve military service. The medical examination includes hearing assessment, conducted by a certified audiologist, in a sealed audio cabin (iac1202ac), with a calibrated manual audiometer (GSI-61). Each ear is examined separately for pure-tone air conduction and bone conduction at 0.5, 1, 2, 4 and 8 kHz. Before 1990, thresholds at 3 kHz and 6 kHz were tested only when hearing deficit was suspected, suggested by evoked response with 25 dB stimulus or higher in the adjacent frequencies. This approach was accepted in civil and military hearing screening programmes in Israel. During the past two decades, however, hearing thresholds at 3 kHz and 6 kHz have been routinely documented in all audiometric examinations. After performing the audiogram, all examinees are examined by an otolaryngologist. Any reversible abnormality affecting hearing is treated, and a repeat audiogram is performed. The primary objective of this study was to assess hearing threshold shifts among Israeli Air Force (IAF) pilots along at least two decades of military flying career, in comparison to those of a non-flying control population—the ATCs. The secondary objective was to evaluate the association between flight hours or different aircraft platforms and hearing acuity, considering age-related hearing loss.
Male aviators from 10 active military squadrons, representing four flying platform, were randomly selected. Flying platforms included fighter jets, combat helicopters, transport helicopters and transport fixed-wing aircraft. Type of aircraft platform each aviator was assigned to was obtained from the IAF personnel database. Medical records were retrieved from the IAFAMC, and pure-tone audiograms at the ages of 25, 30, 35, 40, 45 and 50 years were reviewed. Data from audiometric examinations at 45 and 50 years of age were not available for all pilots. Exclusion criteria included audiograms undertaken due to loss of active flying status for any reason (voluntary or not) but not because of hearing loss. All pilots included had normal hearing at recruitment (no record of hearing impairment in either ear of more than 25 dB in all examined frequencies), required to qualify for the IAF flight academy. Additionally, pilots with chronic otological pathology documented in their medical records were excluded.
The pure-tone audiograms of the pilots were compared with those of air traffic controllers (ATCs), going through similar annual medical check-ups in the IAFAMC. This population was selected as the control group, as their military duties do not involve ambient noise exposure. Inclusion criteria for ATCs were similar as for pilots. We obtained the medical records of only those with documented audiometric follow-up for at least 20 years. Limited number of ATCs has maintained active military reserve duty after turning 40 years old, resulting in a relatively small population of ATCs. The Israel Defence Forces Medical Corps institutional review board approved the study.
Statistical analysis was performed with SPSS (IBM, V.21). Descriptive statistics were reported. A χ2 test was used for comparison of hearing thresholds between different flying platforms, in each age group, and for each ear. A between-subjects analysis of variance (ANOVA) test was applied in analyses combining measurements of both ears. Associations between hearing threshold shifts and military duties (pilots in four different aircraft platforms or ATCs) were evaluated by multiple logistic regression models, with age and flight hours as independent variables. Posthoc comparisons and Scheffe test were used to detect significant differences between the flying platforms and the ATCs. Statistical significance for all tests was considered when the p-value was <0.05.
One hundred and sixty-three pilots were included in the study, distributed to the various aircraft platforms as follows: 54 fighter jet pilots, 27 combat helicopter pilots, 52 transport helicopter pilots and 30 fixed-wing transport aircraft pilots (Table 1). Audiometric data was available up to the age of 40 years for 94% of pilots (n=154). Those examined at the ages of 45 and 50 years constituted 44% (n=72) and 14% (n=23) of pilots, respectively. The control group consisted of 17 ATCs. Audiometric data at 45 and 50 years of age were available for 41% (n=7) and 18% (n=3) of ATCs, respectively.
The mandatory regular military service period for IAF pilot is 12 years, resulting in discharge at the age of 30 years. Thirty-year-old pilots had nearly 3000 hours of flight on average (mean 2872, 95% CI 2108 to 3636). Similar number of flight hours was recorded for 40-year-old pilots (mean 2864, 95% CI 2392 to 3335). Senior pilots, practising military flights until the age of 50 years, had over 4500 flying hours on average (mean 4626, 95% CI 4089 to 5164).
Mean hearing thresholds
The mean hearing thresholds of all aviators and ATCs at the ages of 35 and 40 years are plotted in Figure 1. A marked notch at 4–6 kHz can be seen in the audiometric profile of pilots flying all four platforms. The notch seems less prominent in the audiogram representing mean hearing thresholds of ATCs, especially in the right ears; however, variances between populations did not reach statistical significance. In an attempt to strengthen the power of this comparison, we included both ears data in the between-subjects ANOVA test, but still no significant difference between hearing thresholds of pilots of various aircraft platforms and ATCs was found. Significant differences of mean hearing thresholds between ears were found at 4 kHz (1.624 dB, p<0.001) and at 6 kHz (2.387 dB, p=0.003).
Risk factors for hearing loss
Flying variables: aircraft platform and flight experience
Figure 2 demonstrates the mean audiogram results of 45-year-old pilots and matched ATCs. No statistical difference was found in hearing thresholds for all frequencies between flying platforms or by the number of flight hours undertaken by the pilots.
The effect of ageing on hearing thresholds
Increasing age had a significantly deleterious effect on hearing thresholds (Figure 3). The univariate ANOVA revealed that significant deterioration of pilots’ hearing threshold had occurred already at the age of 30 years in relation to measurements at recruitment but only at 4 kHz (Table 2). It is important to note that thresholds at 0.5 kHz ‘improved’ with ageing, an observation that can be explained by inadequate ambient noise control at the time the recruitment audiogram was performed.
Mean audiometric profiles of IAF pilots, followed along over two decades of active military flight, indicate hearing threshold shifts in the high frequencies range, compatible with noise damage. Slight shifts at 4 kHz were first noted in mean audiograms of 30-year-old pilots, suggesting noise effect rather than age-induced hearing loss. However, no flight variable such as flying platform or hours of flight was associated with deterioration of hearing thresholds, and no significant difference from non-flying controls (ATCs) was seen.
The noise notch
The audiograms in our series demonstrated a pattern, typically considered as the ‘noise notch’, in the range of frequencies most susceptible to noise damage (3–6 kHz).2 This special feature of hearing loss has been described in several other studies, performed on military and civilian transport pilots.4 6 8 10 Raynal et al 6 reported a V-shape centred hearing deficit at 6 kHz in all military aircraft categories pilots. They implicated aircraft noise exposure, since this pattern seems to represent regular exposure to broadband frequencies of continuous noise, also evident in other noise-exposed populations, such as classical orchestral musicians.11 In our study, the difference between mean audiograms of pilots and ATCs (Figure 1) did not reach statistical significance in either ear. Nonetheless, the authors believe that the comparable hearing thresholds of pilots and ATCs could represent satisfying implementation of hearing protection means of IAF pilots. We could not find any type of aircraft more deleterious to hearing than others, in accordance with a previous study from the IAF,3 but opposed to some other reports.6 8
The effect of ageing on hearing
Presbycusis and noise-induced threshold shift are considered additive in the process of hearing loss.12 Most studies on aviators implicated age as a major risk factor for hearing loss,3 4 7 8 13 and some even reported that the effect of age had seemed to superimpose all other factors, such as flight hours and military service years.3 4 Our data revealed a slight (but statistically significant) hearing threshold shift at 4 kHz in 30-year-old pilots, compared with the audiogram at recruitment, whereas significant shifts in other frequencies were noted only later in follow-up audiograms (Table 2). These findings are consistent with previous publications, suggesting the effects of noise exposure are more evident in young subjects.14 The greatest reduction in hearing threshold occurs in the first 6 to 10 years of exposure.12 14–16 It can be speculated that the slight hearing deficit demonstrated at 4 kHz in the audiometric profile of the pilots, at a relatively young age, was noise-induced rather than age-related. However, mean hearing threshold at 4 kHz for age-matched ATCs was not significantly different.
Hearing loss asymmetry
Our results suggesting asymmetry between the ears, with left ear poorer hearing thresholds in high-tone frequencies, are in correspondence with other reports.6 17 The asymmetry could derive from exogenous factors, such as noise asymmetry inside the cockpit or the difference in conduction to both ears through communication devices. Furthermore, there has been evidence, in several publications, of a hearing asymmetry in disfavour of the left ear even if noise exposure is symmetrical.11 A large-scale study on left-handed versus right-handed subjects operating firearms gave the same conclusion.18 Nageris et al 19 reported asymmetric hearing loss in a large group of army personnel exposed to high-intensity noise. No relation to acoustic reflex activity or handedness was found. Interaural threshold differences greater than attributable to measurement error have also been demonstrated in non-noise-exposed UK population.20
We recognise that our study has several potential limitations. First, data on smoking status, chronic illnesses and use of medications were not available, rendering the possibility of confounding variables affecting the results. Other weaknesses of the study are the small sample size of ATCs constituting the control group and the missing measurements at 3 kHz and at 6 kHz in routine audiometric examinations done before 1990 (following the common practice in Israel at the time). We also did not measure direct noise exposure within cockpits of the various aircraft, and information regarding recreational noise exposure is lacking for all study groups.
Noise exposure causes irreversible hearing damage. Increasing awareness to detrimental effect of aircraft noise has led to improved hearing protection measures for military aircrew personnel. Earplugs under headsets or helmets with active noise reduction systems are now standard in the IAF. Our data suggest that the hearing thresholds in IAF pilots may have been affected by noise, as reflected by characteristic noise notch. It could be speculated that the shifts of the hearing thresholds cannot be attributed to ageing alone, since significant changes were noted at a relatively young age. Yet, no variable related to the pilots’ military occupation, such as a certain aircraft platform or hours of flight, was associated with these shifts. Further studies should be conducted to reinforce our findings, taking into consideration other occupational and recreational noise exposures as well as the implementation of advanced hearing protection measures.
The authors would like to thank Dr Ron Kedem for assistance with statistical analysis.
Contributors All authors have made a significant contribution. ZJ and AG conceived and designed the study. HB and LKF collected the data. YFN contributed the materials. LKF, BG, YS and EA analysed the data. LKF wrote the paper. All authors have approved the final draft of the manuscript.
Competing interests None declared.
Ethics approval The Israel Defense Forces Medical Corps institutional review board.
Provenance and peer review Not commissioned; externally peer reviewed.
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