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ARTICLE Table of Contents   
Year : 2009  |  Volume : 11  |  Issue : 44  |  Page : 176-181
Non-auditory health effects among air force crew chiefs exposed to high level sound

1 Danish Defence, Occupational Health Center South, Treldevej 110, DK 7000 Fredericia, Denmark
2 National Research Centre for the Working Environment, Copenhagen, Denmark
3 Danish Armed Forces Health Services, Fighter Wing Skrydstrup, Skrydstrup, Denmark
4 Danish Air Force, Karup, Denmark

Click here for correspondence address and email
Date of Web Publication11-Jul-2009

The possibility of non-auditory health effects in connection with occupational exposure to high level sound is supposed by some researchers, but is still debated. Crew chiefs on airfields are exposed to high-level aircraft sound when working close to aircraft with running engines. We compared their health status with a similar control group who were not subject to this specific sound exposure. Health records of 42 crew chiefs were compared to health records of 42 aircraft mechanics and 17 former crew chiefs. The specific sound exposure of crew chiefs was assessed. The number of reported disease cases was generally small, but generally slightly higher among mechanics than among crew chiefs. Diseases of the ear were more frequent among crew chiefs (not significant). Former crew chiefs reported fewer diseases of the ear and more airways infections (both significant). The sound exposure during launch was up to 144 dB (peak) and 124 dB (L eq ), but for limited time. The study did not reveal a higher disease frequency in general among crew chiefs. However, it did reveal a tendency to ear diseases, possibly due to their exposure to high-level sound.

Keywords: Aircraft noise, exposure-response relationship, hypertension

How to cite this article:
Jensen A, Lund SP, Lucke TH, Clausen OV, Svendsen JT. Non-auditory health effects among air force crew chiefs exposed to high level sound. Noise Health 2009;11:176-81

How to cite this URL:
Jensen A, Lund SP, Lucke TH, Clausen OV, Svendsen JT. Non-auditory health effects among air force crew chiefs exposed to high level sound. Noise Health [serial online] 2009 [cited 2023 Dec 6];11:176-81. Available from: https://www.noiseandhealth.org/text.asp?2009/11/44/176/53365

  Introduction Top

Noise induced hearing loss has been recognized for centuries as an occupational risk. The possibility of non-auditory health effects due to occupational noise has received moderate, but increasing attention in scientific literature. Occupational sound exposure has been associated with hypertension, [1],[2] increased relative risk of myocardial infarction [3] and ischemic heart disease. [4],[5]

The effects of low-frequency sound exposure in the environment have been investigated [6] with a focus on psychological effects of low-level sound at low frequencies. A German literature review [7],[8] concludes that symptoms like fatigue and concentration difficulties can occur in humans exposed to low-frequency sound above the hearing threshold. Changes in the internal organs, i.e. in the heart muscle, were observed in laboratory animal experiments. Experimental studies with rats [9] indicate that infrasound can damage the myocardial ultra-structure. Yet another study [10] showed that similar alterations in the myocardium of rats following an exposure to loud noise could be prevented by treatment with diazepam. Thus, these alterations of the myocardium may be the result of a stress-reaction and activation of the sympathetic nervous system. It has been recognized that pressure waves may produce damage in air-filled organs. [11] Anecdotally, immediate damage to the internal organs [12],[13] after exposure to very high levels of low-frequency sound is described. However, it is not quite clear when and under which conditions sound waves, essentially being pressure waves, may actually cause health effects on the internal organs directly.

A Portuguese research group has published several papers, concluding that exposure to low-frequency sound below 500 Hz and above 90 dB leads to a variety of symptoms and pathological organ changes, [14],[15],[16] which they have grouped together under a common syndrome, Vibro-Acoustic Disease (VAD). Their primary group under study has been airfield ground personnel performing maintenance procedures on aircraft with running engines. The pathologies of VAD are described as proliferation of connective tissue fiber collagen and elastin in the absence of an inflammatory process. The pathologies include collagenous thickening of blood and lymphatic vessel walls, the pericardium, the lungs and the pleura, found together with decreased respiratory drive, but also changes in the respiratory and gastric epithelium, endocrine disorders, late onset epilepsy and auto-immune disease, [17] In essence, the Portuguese studies have associated low-frequency sound exposure with a variety of pathological findings, but have only provided very limited dose assessment. Until now, the relationship between low-frequency sound exposure and the claimed pathological findings has not been substantiated by studies from other research groups. The existence of VAD is still debated and the concept is met with skepticism. [18]

Air force crew chiefs in Denmark are exposed to very high sound levels during launch and recovery procedures, working close to the aircraft with the engine running. Previous assessment exposures of Danish crew chiefs (own data) showed up to 129 dB L eq (lin) during launch procedures of jet fighter aircraft. The possibility of serious health effects through this exposure calls for considerations concerning preventive measures. In some countries crew chiefs wear noise protection clothing [19] as well as hearing protection during these procedures.

On the other hand, the uncertainty about the conclusions of the Portuguese research group makes a specific study of actual health effects among crew chiefs relevant. This is both necessary to make decisions about preventive measures and to be able to inform the crew chiefs correctly about their specific risk. We carried out a controlled study to look for health effects among the highly sound exposed crew chiefs compared to similar personnel without a special noise exposure.

  Materials and Methods Top

Danish jet fighters are based at one airfield in southern Jutland. The airbase has approximately 50 male crew chiefs carrying out launch and recovery procedures of jet fighters with running engines, all similarly exposed to high-level sound during these procedures. No other group of airfield ground personnel has a similar exposure within the Danish Air Force. The 50 crew chiefs, our investigation group, are the complete population of jet fighter crew chiefs in Denmark. The approximately 300 male mechanics at the same airfield are a widely similar group, except they are not subjected to the same sound exposure. This group was used as a control group. In contrast to crew chiefs, the mechanics only work on aircraft while the engines are turned off. Thus, they have a clear exposure contrast compared to the crew chiefs concerning high-level sound. We used existing health files from previous compulsory health examinations, which made the inclusion of all identified crew chiefs and all selected mechanics in the study possible.

Crew chiefs are recruited among the mechanics. Both groups have to fulfill identical health demands prior to employment. Hazardous noise is not expected to be an issue at the work place of the mechanics, and hearing protection is not compulsory equipment when working in these areas. Crew chiefs wear both earmuffs and otoplastics as hearing protection during launch and recovery procedures of aircraft when the engines are running.

We estimated the special noise exposure of the crew chiefs on the basis of available data from the number of launches and new assessments of the noise exposure during launch and recovery procedures of the particular aircraft type. These procedures have been carried out at the base since 1980. The procedures have taken place either in open air or in one of the two types of shelter present at the base. Measurements of the sound level were carried out during these procedures in order to estimate the sound exposure of the crew chiefs. We used a man-borne microphone, attached to the intercom cable of the crew chief [Figure 1] and, as reference, a microphone at a fixed location next to the aircraft. Noise tracks were recorded for later analysis of peak levels (L peak ), average levels (L eq ) and frequency bands on Brel and Kjζr PULSE Front End type 3510-B-130, using the corresponding software PULSE version 12.0. The sound level was measured separately for launch and recovery procedures, and at all three locations, where these procedures take place. When possible, at least two measurements were made for each location to be able to rule out the most obvious artifacts.

From the base administration we received data on the yearly number of launches since 1980 and an estimate of the distribution of launches between the two types of shelter and the open air. For the average crew chief the yearly exposure time to high-level aircraft sound could then be estimated on the basis of the number of launches for the time period, the average number of crew chiefs and the present duration of launch and recovery procedures. All crew chiefs were assumed to have participated equally in these procedures. Taking the seniority into consideration, a cumulated average exposure time to high-level aircraft sound could be roughly estimated for the crew chiefs.

Crew chiefs and mechanics had to pass compulsory health checks at the local infirmary every third year since 1992. The health check consisted of a questionnaire, a urine test, measured data (continuous) on height, weight, pulse and blood pressure, a blood sample for hemoglobin and cholesterol and a spirometry. Finally, a medical interview and an examination were carried out. The questionnaire consisted of simple questions on actual or past health problems, divided into major organ-specific disease groups, plus questions about daily medicine intake, smoking and hospital treatment.

In December 2006, the personnel administration provided personal data on present crew chiefs, present mechanics and crew chiefs who had terminated their duty prior to planned retirement (former crew chiefs). The data included age, seniority in the Danish Defence and also for crew chiefs, seniority in this job function. All present male crew chiefs were included in the investigation group. For each crew chief a mechanic was randomly selected within the same five-year interval of age and seniority in the Danish Defence. These mechanics were included in the control group. Mechanics who had previously worked as crew chiefs were excluded. Attempts were made to retrieve complete health data on former crew chiefs. Those with available health data were all included in a separate group (former crew chiefs). Due to their small number, they could not be matched to the present crew chiefs.

Health files from the compulsory health checks were retrieved for all three groups (present and former crew chiefs and mechanics). Data from the most recent health examination was compared between the investigation group and the control group, and between present and former crew chiefs. Data consisted of categorical data, mainly self-reported data from a questionnaire about the presence or absence of health problems within a major disease group. A sum of positive answers for earlier or present disease for each specific disease group, for daily medicine intake, smoking, hospital treatment or traces of blood in urine could be calculated for each of the three groups. Further data was continuous data from biological measuring. For each of these data groups, including age, seniority, blood samples and spirometry, an average was calculated for each group. For spirometry, the indicated percentage of normal values related to age and height were used for statistical analysis. Medical remarks from the health checks were checked for consistency with the self-reported data and the measured data.

Statistical analysis was carried out using Mantel-Haenszel's X2 -test for the categorical data and the unpaired Student's t- test ( www.physics.csbsju.edu ) for the continuous data. Differences were regarded as significant when the P value was below 0.05.

  Results Top

In December 2006, 49 male crew chiefs could be identified in the available personnel data. A control group of 49 matching male mechanics was established. The following data collection revealed that seven crew chiefs and four aircraft mechanics in the meantime had left the job, and three mechanics had earlier worked as crew chiefs. Thus, the final groups included 42 crew chiefs and 42 mechanics. The average age of the crew chiefs was 47.8 years and 45.8 years for the mechanics. Mean seniority of the crew chiefs in the Danish Defence was 26,7 years and 24.1 years for the mechanics, and mean seniority as crew chief was 19.6 years. Only four had seniority as crew chief of 10 years or less.

Thirty-one former crew chiefs were found in the available data, and with the seven former crew chiefs mentioned above, a total of 38 former crew chiefs were identified. Health data could only be retrieved from 17 of those. No data on spirometry or seniority were available for the former crew chiefs. The average age of the 17 earlier crew chiefs was 41.5 years.

The sound exposure during launch and recovery procedures was measured for the frequency band 0.1 Hz - 20 kHz. Exposure levels [Table 1] were analyzed for the complete frequency band and for low-frequency bands similar to those used by other research groups. [14,20] We found peaks reaching 144 dB (lin) and L eq levels up to 124 dB (lin). A frequency analysis showed the highest sound pressure levels at 2 to 4 kHz. In the shelters high levels were also registered at a frequency of 2.5 Hz, which represents their resonance frequency (own data from earlier noise assessments). The present procedures on the airbase did not include regular launch or recovery procedures in open air. Thus, only one measuring during launch and none during recovery in open air could be made. The technical details of the noise measurements are described in a separate measuring protocol (own data).

Assuming an average of 50 active crew chiefs, each participating equally in the launch and recovery procedures, the average number of launch and recovery procedures for each crew chief per year was calculated from the total number of launches from the air base over the years [Table 2]. The estimated yearly duration of high-level sound exposure for each crew chief was between 29 hours (1981-1995) and 19 hours (2001-2005) assuming the same duration of the sound exposure during each launch or recovery procedure as now measured. Thus, the crew chiefs each had an average total duration of high-level sound exposure of approximately 470 hours during the 19.6 years of average duty. The sound level could have been slightly lower in the years before 1995 than after 1995 due to the higher relative number of launches and recoveries in open air.

The self-reported categorical health data of the crew chiefs and the mechanics [Table 3] did not reveal any significant differences, although there was a higher frequency of ear diseases, including hearing loss, among the crew chiefs. For nearly all other diseases, the health effects reported had a slightly higher frequency among the aircraft mechanics than among crew chiefs. Overall, there were no obvious differences between crew chiefs and mechanics concerning the continuous test results.

Former crew chiefs had a significantly higher rate of reported "frequent airway infections" and a significantly lower rate of "diseases of the ear" than the present crew chiefs. No former crew chief reported daily medicine intake. No further differences between former and present crew chiefs were seen concerning the continuous test results [Table 4].

According to the notes of the physician who carried out the health examinations, there were cases of a variety of specific diagnosis in all groups, but with no obvious pattern of disease among either group. Apart from hypertension, no specific diagnosis occurred more than once. Four crew chiefs and three mechanics had hypertension.

  Discussion Top

The present study was undertaken in order to evaluate the potential risk of health effects among crew chiefs due to high sound levels during launch and recovery of jet aircraft fighters. The study was carried out on all available Danish jet fighter crew chiefs using existing health data. This leads to major shortcomings, mainly the small sample size and relatively unspecific health data. Even though the existence of a VAD is discussed, controlled epidemiological studies dealing with this issue have not yet been published. All literature on VAD is practically from one research group. Results from studies on other types of sound exposure, i.e. traffic noise or noise around airports, are of limited value in this connection due to the different character of the exposure. The overall need for knowledge on health effects due to high-level sound exposure encouraged us to carry out this study despite the shortcomings.

We found a higher frequency of ear diseases among crew chiefs than among mechanics, although not significant. This probably reflects a higher risk of hearing loss induced by noise exposure. Anecdotally, one crew chief had terminated his duty due to tinnitus, indicating that noise is a health issue among the crew chiefs. Otherwise there is no indication of crew chiefs having a higher disease risk than mechanics.

The main finding among the former crew chiefs compared to the present crew chiefs is the statistically significant higher number of reported "frequent airway infections". This symptom is mentioned in connection with the VAD syndrome. [14] Without the possibility to interview the former crew chiefs, it cannot be ruled out that some crew chiefs terminated their duty due to airways infections, thereby creating a healthy worker effect. However, if airway infections are a major issue among crew chiefs, a higher number of reported cases should also have appeared among the present crew chiefs.

In epidemiology, using health data from regular health checks from the past has its disadvantages. The examination protocol for these health checks is not specific to the purpose of the investigation. The data are neither as detailed nor as sensitive as data from specific test results could be. However, the health examinations were carried out with the same protocol for all participants in the study. Using already existing health data made it possible to retrieve data from all present crew chiefs, without any non-responders. Furthermore, health data from previous health examinations has the advantage of not being biased. Neither the examining physician nor the examined employee knew that the collected data would be used for this study later on.

Specific health examinations might have revealed specific health effects among crew chiefs. Echocardiography has been used by the Portuguese research group to detect thickening of the pericardium, [21] but other experts have claimed this examination to be without the necessary sensitivity to detect changes of the described magnitude. [22] For ethical reasons, invasive diagnostic methods do not seem to be appropriate in a healthy investigation group. Without distinct health effects in the present data it is difficult to point out further reasonable diagnostic methods that could reveal differences between the exposed group and the control group.

The possibility of exposure to other hazards than noise, including chemicals, exists for both the crew chiefs and the aircraft mechanics. Exposure to hazardous chemicals is, however, sporadic for both groups, and is not considerably different between the groups. We have no knowledge of other relevant hazards in the two groups.

Crew chiefs are exposed to high-level sound during launch and recovery procedures, also when the exposure to the lower frequencies is regarded separately. On the other hand, the exposure takes place over limited periods of time and the duration can be roughly calculated to be less than 2% of the total time on duty. We have no reliable data on the exact number of launch and recovery procedures for each crew chief and the actual duration of these procedures in the past. Procedures might have changed over the years and different crew chiefs might do things in a slightly different way, thereby resulting in variations of the sound exposure. Thus, the individual sound exposure is only a rough estimate, based on assumptions. Crew chiefs generally have a long seniority. The mean seniority as crew chief was 19.6 years, and only four had a seniority of less than 10 years. Considering the rather uniform seniority, the relatively small number of reported health effects and the lack of distinct differences between the groups, we believe it makes little sense to attempt to establish a dose-response relationship. Both crew chiefs and mechanics are exposed to usual sound at different levels at the work place as well as off duty. According to the responsible manager, repairing jet fighters does not include noisy procedures, apart from test procedures on jet engines. This takes place in special noise insulated locations. We therefore believe that the sound exposure during launch and recovery procedures is the overall difference in the sound exposure pattern between crew chiefs and mechanics, and that there is a distinctive exposure contrast between the two groups.

Conclusively, we believe that our findings do not support the hypothesis of a specific syndrome in relation to long-term exposure to low-frequency sound fields, described as VAD.[14] Certainly, we cannot exclude completely that more specific health effects could have been revealed by special diagnostic tests. There do not seem to be immediate indications for such tests on the basis of our results. Controlled epidemiological studies on a larger scale would be useful to be able to estimate the risk on a more reliable basis.

  References Top

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Correspondence Address:
Anker Jensen
Danish Defence, Occupational Health Center South, Treldevej 110, DK 7000 Fredericia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1463-1741.53365

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

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