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  Table of Contents    
Year : 2011  |  Volume : 13  |  Issue : 54  |  Page : 364-370
Hearing status among aircraft maintenance personnel in a commercial airline company

1 Department of Medical Sciences/Occupational and Environmental Medicine, Uppsala University, Uppsala University Hospital, SE-751 85 Uppsala, Sweden
2 Department of Aviation Medicine (HMS), Scandinavian Airlines System (SAS), SE-195 87 Stockholm, Sweden

Click here for correspondence address and email
Date of Web Publication28-Sep-2011

The aim was to study subjective and objective hearing loss in a population of aircraft maintenance workers and identify predictors. A total of 327 aircraft maintenance personnel answered a self-administered work environment questionnaire (response rate 76%) and underwent audiometric test. The mean values for the hearing threshold at 3, 4, and 6 kHz for the ear with the most hearing loss were compared with a Swedish population database of persons not occupationally exposed to noise. Equivalent noise exposure during a working day was measured. Relationships between subjective and objective hearing loss and possible predictors (age, years of employment, self-reported exposure to solvents, blood pressure, and psycho-social factors) were analyzed by multiple logistic regression. At younger ages (<40 years), aircraft maintenance workers had higher hearing thresholds (1-3 dB) compared to the reference group, but such a difference was not found in older employees. Relationships were found between age and objective hearing loss, and between exposure to solvents and reported subjective hearing loss. Equivalent noise exposure during working days were 70-91 dB(A) with a maximal noise level of 119 dB(A). Aircraft maintenance workers are exposed to equivalent noise levels above the Swedish occupational standard, including some very high peak exposures. Younger employees have a higher age-matched hearing threshold level compared with a reference group. Thus, there is a need for further preventive measures.

Keywords: Aircraft maintenance personnel, aircraft, hearing loss, hearing threshold levels, noise

How to cite this article:
Smedje G, Lundén M, Gärtner L, Lundgren H, Lindgren T. Hearing status among aircraft maintenance personnel in a commercial airline company. Noise Health 2011;13:364-70

How to cite this URL:
Smedje G, Lundén M, Gärtner L, Lundgren H, Lindgren T. Hearing status among aircraft maintenance personnel in a commercial airline company. Noise Health [serial online] 2011 [cited 2023 Dec 1];13:364-70. Available from: https://www.noiseandhealth.org/text.asp?2011/13/54/364/85509

  Introduction Top

Risk factors for hearing loss include aging, genetic heredity, head injury, infections, certain drugs, high blood pressure, tobacco smoking, and noise, both occupational and during leisure activities. [1],[2],[3],[4],[5],[6] In the aviation industry high noise levels are prevalent and the hearing ability of airline employees are usually followed by repeated audiometric tests. We have previously published data on hearing status and noise exposure in pilots and cabin crew. [7],[8] For both groups we concluded that they were exposed to equivalent noise levels below the occupational standard of 85 dB(A), and they had normal age-matched hearing threshold levels. Another noise exposed group is the aircraft maintenance workers. Noise exposure occurs during maintenance work in hangar and on station apron during take-off and landing. [9] A Chinese study [10] investigated hearing loss in aircraft maintenance workers, airport firemen, airport policemen, airline ground staff, and airport civil servants. The prevalence of a high-frequency loss was 42%, with the highest prevalence in maintenance workers (65%) and firemen (55%). Hong and Kim [11] identified risk factors as noise exposure level, years of noise exposure, nonoccupational noise exposure, history of ear disease, ototoxic drug use, cigarette smoking, hypertension, and use of hearing protective device. Aircraft maintenance personnel are also exposed to jet fuels and solvents that may have detrimental effects on hearing [12] and the combination of noise exposure and exposure to solvents may increase the risk. Subjects with noise and jet fuel exposure had an increase in hearing loss, even at fuel exposure estimates well below the threshold limit values "and the effect of jet fuel appeared to be stronger at shorter duration of exposure." [13] Kim et al. [14] studied the effect of occupational exposure to noise and organic solvents on hearing loss in male aviation industry workers. Pure tone audiometry from the workers' biannual medical surveillance was used to assess hearing loss. Controlling for age, and compared with the unexposed, the relative risk of hearing loss was 8.1 in the group exposed to both noise and solvents, 4.3 in the noise-only group and 2.6 in the solvents-only group. Prasher et al. [15] compared aircraft maintenance workers with workers exposed to noise alone, to solvents alone, and to nonexposed. There was a significant effect on pure tone thresholds for both the noise and the solvents + noise groups, but the mean acoustic reflex thresholds showed a pattern which differentiated the noise from the solvent and noise groups.

However, the studies on noise and hearing loss in aircraft maintenance workers are still few. Furthermore, they are rarely investigating the long time effect by studying hearing loss in those with many years of work experience.

Hearing loss is well documented through many studies and there is an international, standardized database for hearing threshold levels as a function of age, ISO 7029, [16] describing hearing threshold levels for an otologically normal population. In ISO 1999, [17] populations in the USA are described. These reference databases have been used as reference populations in previous studies. [18],[19] In addition, there is a Swedish database of hearing threshold levels for a population without occupational noise exposure. [20] The Swedish database does not exclude individuals with prior ear pathology or other nonoccupational factors that may influence hearing. Thus it may be suitable when studying hearing loss in an otologically unscreened population, in relation to age and occupational noise.

The aim of this study was to investigate objective and subjective hearing ability in aircraft maintenance workers at a Swedish airline company and to identify work-related risk factors for hearing loss, such as exposure to noise, occupation, years of employment, and self-reported exposure to solvents.

  Study Population and Methods Top

Study population

All maintenance personnel employed in a Swedish airline company who had undergone a voluntary audiometric test at the company's occupational health service (OHS) in the years 2008-2009 were included. They were 327 males and 9 females, with a range of age of 22-67 years old. Due to the small number of females, they were excluded from further analysis. In total, there were 440 maintenance workers employed during this time period; thus the participation rate was 76%.

Maintainers were divided into six occupations: (1) technicians following the aircraft during in and out taxiing and working on station apron as well as in hangar with aircraft, (2) mechanics working only in hangar with aircraft, (3) sheet-metal workers working in hangar using riveting hammer, (4) electricians, working only in hangar, (5) supply work in hangar, (6) administration work. Work in hangar means stripping down and mounting parts of the aircraft. Both technicians and mechanics work in the same area with similar tasks but technicians have higher education with the responsibility of performed tasks. Administration work in hangar is usually performed by individuals that have previously been working as technicians or mechanics.

For evaluation of audiometric tests, data from the maintenance workers were compared with a large Swedish database on hearing thresholds at different ages. [20] This reference population consists of 266 males randomly selected from the population census in one mid-Swedish county (20-79 years), excluding subjects exposed to hearing damaging noise levels at work. For this reference group, hearing thresholds were determined by the manual ascending method.


A self-administered questionnaire was mailed by the OHS in November 2008-May 2009. The questionnaire was based on a standardized questionnaire developed by the OHS, and has been used in the company for many years. The questionnaire contained one question on subjective hearing loss, asking "Do you have a problem hearing?" There was one question on work environmental items: "Are you bothered by any of the following factors in your work?," followed by a list of factors including "solvents/chemicals." Another two questions asked about the psychosocial work environment: "Do you have too much to do at work?" and "Does your work demand too much of you - like too great responsibility, too difficult work tasks, unclear work tasks?" For each of these questions, there were four alternatives: "no, never," "no seldom," "yes, sometimes," and "yes, often." There was also one question asking "Have you ever had, or do you have, a high blood pressure?" (yes, no).

Audiometric test

The audiometric tests were carried out using a Bekesy audiometer model BA2 (Interacoustics A/S, DK-5610 Assens, Denmark), calibrated annually according to international standards. [21] A sound-attenuating chamber was used with a background sound pressure levels not exceeding the maximum octave-band levels for audiometric test rooms. [22] In this test, the subject presses a button when he hears an automatically generated tone, and threshold levels were determined according to the ascending method. Each subject was instructed and surveyed by a nurse experienced in audiometric testing. The audiometric tests were performed at random times during the work shift.

Hearing ability of the high-frequency pure-tone of 3, 4, and 6 kHz was used to evaluate hearing loss due to occupational noise exposure, as previously described. [23] In the evaluation, we calculated the mean value at 3, 4, and 6 kHz for the ear with the worst hearing ability, for each individual. This value is entitled HF PTA WE (High Frequency Pure Tone Average for the Worse Ear). The worse ear was used as it is a means of early detection of a developing hearing loss. The median values for HF PTA WE were calculated for each age and was compared with the percentile values for the reference population.

In the reference material, a 10-year window of the age-related data is used to calculate smooth percentile curves from the 10 th to the 90 th percentile. Johansson and Arlinger [24] suggests that the 60 th percentile of the PTA distribution would be a reasonable criterion value for evaluating the influence from noise exposure on a group level. This criterion is based on an assumption of normal distribution, with 50% of the population above the 50 th percentile, and empirical data showing that the standard deviation (SD) equals the 84 th percentile.

Noise exposure

Exposure measurements were performed in 2009 by an experienced industrial hygienist, using a Spark 706RC noise dosimeter, (Larson Davis INC, UT 84601, USA). Equivalent personal exposure (Leq) dB(A) during a working day was measured during different working tasks. The microphone was placed at the workers' side within 100 mm of the ear where noise was normally received. Technicians and mechanics that follow the aircraft on station apron use ear protection devices (earmuff) with communication, and noise exposures were also measured inside the earmuff. In total, exposure measurements were performed during two workdays on station apron, two workdays in hangar and one workday while working in the hangar with riveting hammer. Furthermore, peak exposures were measured at certain situations, using noise analyzer Norsonic type 110 (Norsonic A, Tranby, Lier, Norway).

Audiometric tests and the questionnaire study were performed by the OHS, as integrated parts of its regular commission to follow the health of the employees. Information on age, date of employment, gender, and audiometric test result was obtained from the medical records. The study was approved by the Ethical review board of Uppsala. Participation was voluntary and the aircraft maintenance workers gave their informed consent.

Statistical analysis

The associations between hearing problems and different predictors were analyzed by univariate analyses and logistic regression using statistical package for social sciences (SPSS). Differences between occupations as regards prevalence of hearing loss and specific questionnaire data were analyzed by Pearson's Chi-square analysis and for demographic data Anova was used. Logistic regression analyses were performed with and without controlling for possible confounders. Two types of outcomes were studied, objective hearing loss according to audiometry and subjective hearing loss as reported by a questionnaire. Correlation between objective and subjective hearing loss was analyzed by Spearman's rho. Measured hearing loss was dichotomized, defining subjects with HF PTA WE above 20 dB as cases and equal and below 20 dB as noncases. The multiple regression model included age, total length of employment, high blood pressure, exposure to solvents, and occupation. When analyzing subjective hearing loss, the answers were dichotomized, defining "often" or "sometimes" as cases and "never" or "seldom" as noncases. This model included age, length of employment, high blood pressure, exposure to solvents, the two questions on psychosocial work environment, and occupation. All questions with four alternatives were dichotomized with "often" or "sometimes" as yes and "seldom" or "never" as no. The choice of which variables to include in the multiple models was based on the prior knowledge on which factors could affect hearing status. Due to collinearity between age and year of employment, analyses were also performed including only one of these factors in the models.

For the logistic regression, odds ratios (OR) 95% and confidence interval (CI) were calculated. In all analyses, a P-value of <0.05 (two-tailed) was considered statistically significant.

  Results Top

Hearing loss and questionnaire data

The prevalence of hearing loss, answers to selected questionnaire data, and some demographic data are given in [Table 1]. Mean age of the employees was 47 years and mean years of employment was 22 years. Mechanics, and those working with supply work in hangar, were somewhat younger and had a shorter duration of employment.
Table 1: Prevalence of hearing loss and demographic data

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The prevalence of hearing loss more than 20 dB in worst ear was 41%, and 34% reported subjective hearing loss. Objective and subjective hearing loss were equally common cross the different occupations.

The prevalence of exposure to solvents was 31%. High blood pressure was reported by 17%. Supply workers and those working in hangar with administration reported less exposure to solvents, and sheet-metal workers more often had a high blood pressure. In total, 77% reported work stress and 35% reported too high work demands with no significant differences between the occupations.

When comparing the result from the audiometries we found that younger (< 40 years) maintenance workers had more hearing loss in worse ear compared with the reference group, with average median values being above the 60th percentiles for the reference material [Figure 1]. The difference was biggest for the age group 35-40 years old, about 3 dB.
Figure 1: Median of HF PTA WE (mean of 3, 4, and 6 kHz) in different age groups for male aircraft maintenance workers compared with 50th, 60th, 70th percentile curves for a reference population

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Regression analyses are presented in [Table 2] and [Table 3]. Univariate analyses showed significant relationships between having more than 20 dB(A) hearing loss and age, and number of years of employment, but not with having a high blood pressure, exposure to solvents or occupation. Multiple logistic regression analysis showed similar results [Table 2]. For subjective hearing loss, significant relationships were found to age and years of employment, but also to high blood pressure, in the univariate analyses [Table 3]. There was also a borderline significance for exposure to solvents (P=0.083), and having too much work to do (P=0.094). In the multiple regression analysis, however, only exposure to solvents was significantly related to subjective hearing loss, with number of years of employment having a borderline significance of P=0.083. Furthermore, the correlation between measured and reported hearing loss was significant (P<0.000, 2-tailed) with a correlation coefficient of 0.401.
Table 2: Relationships between at least 20 dB(A) hearing loss and some possible predictors. Odds ratios with 95% confidence interval

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Table 3: Relationships between subjective hearing loss and some possible predictors. Odds ratios with 95% confidence intervals

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In the multiple regression models, there were collinearity problems between age and years of employment but models excluding either of these gave similar results (data not shown). No major differences were observed between different occupations. Moreover, in the regression models we found no influence of blood pressure, and psychosocial work environment, on measured or reported hearing loss.

Noise exposure

The noise exposure measurements showed that the Leq measured while working in hangar was between 70 and 91 dB(A), and when following the aircraft on the station apron it was 81 dB(A), during an assumed 8-hour workday [Table 4].However, peak exposures could be very high. During the most noisy work tasks ear protection was usually used. The highest exposure levels occurred during sheet-metal work and while using riveting hammers in hangar, which happens 30 seconds to 3 hours per working day. While working with the riveting hammer, the highest maximum sound pressure level, measured inside the ear protection device, was 119 dB(A). Other workers present in hangar are also affected by this noise; however they do not usually wear any ear protection device. While following aircraft in and out from the gate, the noise level inside earmuff was 87 dB(A) during operation with a maximum sound pressure level of 93 dB(A). For those who follow the aircraft on station apron during in-out taxiing this task happens 8-12 times per working day. Sheet-metal workers had the highest noise exposure, followed by technicians. Two examples of exposure curves are given in [Figure 2] and [Figure 3].
Table 4: Equivalent noise levels measured inside and outside ear protection device

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Figure 2: Exposure to noise, Leq during operation, working in hangar with sheet-metal and riveting hammer, measured outside earmuff. The exposure corresponds to 92 dB(A) during an 8-hour working day

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Figure 3: Exposure to noise, Leq during operation, working in hangar with stripping down and mounting parts of the aircraft, without use of earmuff. The exposure corresponds to 70 dB(A) during an 8-hour working day

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  Discussion and Conclusions Top

We found that younger aircraft technicians and mechanics in a Swedish commercial airline company had a higher rate of hearing loss in the worse ear than a non-noise exposed reference population from the same country. However, at older age there was no difference in hearing ability. We also found that subjective hearing loss was associated with self-reported exposure to solvents.

The hearing test was not compulsory, but the participation rate was relatively high (76%); thus we do not expect any major selection bias and the participants should be representative of the source population. They were compared with a reference group consisting of a random sample from the general population, screened for not being exposed to occupational noise. A selection bias, with more healthy individuals in the group of maintenance workers compared to in the reference group (healthy worker effect, HWE), could be possible. [25] However, the younger maintenance workers had a higher hearing threshold level than the reference group, implying that such a selection bias is of minor importance in the present study. Socio-demographic differences between the study group and the reference population could also be of importance, but we have no details on such factors. However, aircraft maintenance workers consist of different occupational groups with different levels of education, and together they would pretty much reflect the average (male) population.

There were also some differences in the methodology of performing the audiometries between the present study and the reference population. The present study used a semiautomatic self-recording Bekesy-audiometer, while the reference population used the manual ascending pure-tone method to determine hearing thresholds. In Sweden, Bekesy audiometry is widely used by occupational health services for screening hearing threshold levels, and is regarded as an easy and effective method for early detection of noise related hearing impairment. In a pure-tone investigation obtained hearing thresholds are higher than in a Bekesy investigation. [26] This may imply that the hearing loss among the young aircraft workers was even higher than recorded.

The audiometric tests were performed at random times during the work shift. As a temporary higher hearing threshold may occur after noise exposure, measured hearing thresholds may be slightly higher than if audiometry had been performed before work. However, there should be no difference in this respect between workers of different age.

In other occupational settings, equivalent noise levels above 85 dB(A) during 8 hours have been shown to increase the risk for hearing loss [27] and the Swedish workers protection standard requires that exposure should not exceed this level. [28] Our noise measurements indicate that some occupations are exposed to equivalent A-weighted and maximal sound pressure level just above this limit value, but the mean exposure level was 81 dB(A). Thus, the higher hearing loss among the younger, especially the 35-40 years old, is not easily explained. During the last years this airline company, like many others, has had to reduce the number of employees, which has resulted in fewer persons in the youngest age groups. However, the study still included 68 workers younger than 40 years old; thus the result being a chance finding seems unlikely. Smoking habits and high blood pressure are risk factors for hearing loss. We have no data on smoking habits in our material but smoking frequency in the general population in Sweden is low (16%, Swedish National Institute of Public Health) and a high blood pressure is unlikely in these young age groups. In recent years it has been suggested that listening to loud music could impair hearing in adolescents and young adults. [29],[30] Furthermore, the rate of noise-induced hearing loss during the first 10 years of exposure has been discussed, [31] suggesting both higher and lower increases of hearing loss during this period compared with, e.g., the ISO-1999 model. However, in the present study, although all age-groups <40 years had a higher mean HF PTA WE than the reference population, it was most pronounced in the oldest group of 35-40 years old, having the longest duration of employment. This makes explanations like entering the work force with a hearing loss or an especially rapid development during the first years of exposure less likely.

We speculate that the observed difference is due to noise exposure at work and that peak exposures might be of importance. While working with riveting hammer the maximum noise level measured inside earmuff, still exceeded the Swedish occupational standard, 115 dB(A). More effective hear protection devices and an increased use of earmuffs also by those working nearby is warranted.

From around the age of 40, the maintenance personnel had similar hearing ability as the reference population. This result might indicate another aspect of the healthy worker effect, that workers who develop health problems are more likely to leave work. [24] One could speculate that hearing ability is worsened with age, due to occupational exposure, and that around 40 those with the worse hearing ability tend to leave.

We found no relationship between hearing loss and occupation. However, within the group of maintenance workers, change of occupation is common, and the present occupation may not always be representative of the individual's cumulative noise exposure. Furthermore, noise exposure in hangar is similar between occupations.

We found exposure to solvents was related to subjective hearing loss. Both these factors were self-reported and a reporting bias could be apprehended. However, we also found a significant correlation between subjective and objective hearing loss; thus such reporting bias might not be substantial.

In conclusion, aircraft maintenance workers <40 years had more hearing loss compared with a reference population not exposed to occupational noise. Noise exposure levels were just above the current Swedish occupational standards, and included some very high peak exposures, despite frequent use of hearing protection devices. There is a need of further preventive measures.

  References Top

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Correspondence Address:
Greta Smedje
Department of Medical Sciences/Occupational and Environmental Medicine, Uppsala University, University Hospital, SE-751 85 Uppsala
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Source of Support: County Council of Uppsala, Sweden, Conflict of Interest: None

DOI: 10.4103/1463-1741.85509

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  [Figure 1], [Figure 2], [Figure 3]

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

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