Annals of Occupational Hygiene Advance Access originally published online on July 7, 2004
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Ann. occup. Hyg., Vol. 48, No. 5, pp. 475-481, 2004
© 2004 British Occupational Hygiene Society
Published by Oxford University Press
Noise Exposure During Alpine Helicopter Rescue Operations
1 Institute of Aerospace Medicine, Faculty of Medicine, Technical University of Aachen, Kullenhofstrasse 50, D-52057 Aachen, Germany; 2 Department of Nephrology and Rheumatology; University of Göttingen, Göttingen, Germany; 3 State Institute for Occupational Health and Safety of North Rhine-Westphalia, Düsseldorf, Germany
Received 9 May 2003; in final form 28 November 2003; published online on 7 July 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Objectives: We estimated the noise exposure of crews working in alpine helicopter rescue systems. Methods: Noise levels of the the helicopters used (Alouette III, Alouette II ‘Lama’, Ecureuil and BK 117) were measured with a device according to class 2 DIN IEC 651. These data were combined with the flight data of the personnel to evaluate the equivalent noise level according to DIN 45645-2. Results and conclusions: While the risk to patients should be limited to temporary threshold shifts the crew members are regularly exposed to equivalent noise levels of >85 dB(A) and, therefore, are at risk of permanent threshold shifts. Consequences for crew fitness to fly and for noise prevention (crew and patients) are discussed.
Keywords: helicopter rescue operations; noise prevention; occupational medicine
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The risk of hearing loss is well established for a wide spectrum of occupations, but to date there are no data for helicopter rescue crews. In the alpine environment these operations differ from other exposure to occupational noise by at least two important factors: (i) extreme variability with some days involving no exposure but others extreme exposure; (ii) very high noise levels with limited protection during work outside the aircraft. The latter is caused by operational tactics like so-called ‘hot loading’, where material or the patient is loaded into the hovering helicopter or, more often, at the beginning of rescue and medical aid before the engine is shut down. Another situation with longer exposure outside the aircraft is winch operation. Here the aircraft cannot be re-entered together with the patient in flight and, therefore, both the rescuer and his patient must stay in the noisy environment outside the aircraft until an intermediate landing place is reached.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Noise levels were measured at typical points inside and outside the aircraft as indicated in Figs 1 and 2 (Alouette III and Alouette II ‘Lama’). The points for Ecureuil and Bk 117 were similar to those for Alouette II and III. These points were chosen because they represent the positions of crew members near the aircraft during rescue operations. At every position at least three independent measures on different flights were taken for at least 1 min each. Data were stored as noise level (dB). For calculation of the equivalent noise level (see below) the average of these noise levels was used if their difference was <5%.
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The microphone was a capacitor microphone (Type 4135; Brüel and Kjaer). The signal was digitally stored according to DIN IEC 651. The system was switched to ‘fast’ mode and assessment was dB(A) (NN, 1994). The system was calibrated according to DIN IEC 651 using a sound calibrator type 4230 (Brüel and Kjaer series no. 1511608) at 94 dB and 1000 Hz. This design corresponds to class 2 DIN IEC 651. The measurements during winch operations were performed 30–200 m above ground to exclude ground effects, e.g. echo. To exclude errors in measurement the microphone was covered against downwash from the rotor during these measurements. This covering device, constructed by the manufactorer of the microphone, is made of soft foam material. Data acquisition inside the aircraft was performed during constant straight flight. At each point of measurement the microphone was held directly beside of the ear of a person working or sitting at the positions as marked in Figs 1 and 2. For winch operations this was accomplished by a long cable hanging down from the helicopter. All measurements were carried out during real rescue operations in an alpine environment.
Data acquisition was performed with a DTC 1000 ES data recorder (Sony). For data evaluation BAS system v. 3.30 (Head Acoustics, Aachen) was used. In accordance with DIN 45645-2, noise exposure was related to a period of 8 h (equivalent noise level; NN, 1997). In cases of extreme variation in noise level, exposure should be calculated for periods of constant noise level. We measured noise levels for the following periods: flight duration (to the site of the accident, to the hospital and back to base) and duration of winch operations. These data were obtained by the pilot’s flight reports of the operations. The time at site of the accident (treatment of the patient) and the time between operations were defined as 60 dB(A), which is the noise level of normal talking. For any flight (period of operation), per day and per base, the calculations were performed with the noise levels obtained as described above, to obtain the equivalent noise level for any day and any crew for the period of investigation.
To calculate the exposure of the personnel the rescue operations of four bases were analyzed over 1 yr [total, 2726: Switzerland, Zermatt (n = 622) and Raron (n = 457); Austria, Landeck (n = 836) and Innsbruck (n = 811)].
In a similar way the noise exposure of the patients was estimated. Additional calculations with the operation’s data but the noise level of a less noisier helicopter (e.g. BK 117) estimate the benefit of noise protection of the personnel if such aircraft are used.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Although the situation and environment of data acquisition were very unusual the results are reproducible with variations of <4%. In the Alouette III the noise levels measured outside the aircraft were 114.9–120.8 dB(A), with the highest levels at the points (H) ‘refueling’ and (G) ‘loading the back of the material basket’, both points near the engine (Figs 1 and 2). The lowest noise level was measured at ‘loading the front of the material basket’ (F in Figs 1 and 2). The other external points, ‘PAX loading’ and ‘winch’, showed 117–118 dB(A). There was a difference of 6 dB(A) between the two possible positions of the patient [head under (C in Figs 1 and 2) or beside (B in Figs 1 and 2) the aircraft] with lower noise level, if the patient’s head was under the aircraft. Because of the overlap of the ranges of the several results obtained at different points (no significant differences) the noise exposure outside the helicopter was assumed to be comparable at these points. Because the highest levels dominate the total exposure of an 8 h day and the personnel are regularly exposed to these levels during hot loading and the beginning and the end of winch operations, 120 dB(A) was used to calculate the exposure outside the aircraft. Inside the aircraft there were noise levels of 104.6–106.5 dB(A), which did not differ significantly. Therefore, these points were assumed to show comparable noise levels and 105 dB(A) was used to calculate the exposure inside the aircraft.
For the Alouette II ‘Lama’ the external noise levels were nearly identical to those for the Alouette III, and 120 dB(A) was used for further calculations. Inside the aircraft levels were 106–109 dB(A). For further calculations 108 dB(A) was used. Outside the Ecureul levels were 109–111 dB(A), dependent on the position. Inside the aircraft levels were 99–101 dB(A). For calculations, 110 and 100 dB(A) were used.
In Switzerland most operations were performed with Alouette III and Alouette II aircraft, in Austria nearly all were done with Ecureuil aircraft. At Raron 42.3% of all days were without noise exposure, at Zermatt 36.3%, at Landeck 31.1% and at Innsbruck 15.8%. There were significantly more days of exposure in Austria (P < 0.001), with Innsbruck showing most days of exposure (P < 0.001). On average the duration of noise exposure on days with rescue operations was 32.8 min (±21.8, range 2–158; Fig. 3). Because of regional characteristics, such as the type of helicopter and the distance to main accident regions or hospitals, there are significant differences in the distribution of daily exposure times between the bases. While the bases at Zermatt and Raron (both in Swizerland) show most days with noise exposure of 21–30 min, at Landeck and Innsbruck (both in Austria) more days with longer exposures (31–60 min) could be found (P < 0.01; Fig. 4a and b).
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Nearly every day with rescue operations showed noise exposure equivalent to a working day of 8 h higher than 85 dB(A) (Table 1). Exposure was between 90 and 105 dB(A) on 52.8% of all days in Switzerland and 62.5% of days in Austria (Table 1). The exposure was >105 dB(A) on 5.2% of days in Switzerland and 10.7% of days in Austria. Although the type of aircraft mostly used in Austria is less noisy than the types investigated in Switzerland, the exposure of the crews does not differ significantly because of the longer flights, which mean longer noise exposures per day in Austria. The exposure of the different bases is shown in Table 1.
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The noise exposure of the patients, calculated in a similar way to those of the crew, is shown in Fig. 5. A total of 67.1% of all patients were exposed to 8 h noise levels of 85–95 dB(A), with 2.8% exposed to up to 110 dB(A).
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To estimate the exposure for helicopter BK 117 a noise level of 94 dB(A) inside and 108 dB(A) outside the aircraft was used. These results were obtained with the same equipment and the same calculations as those of the real operations. If all operations had been performed with this helicopter, the personnel would have been significantly less exposed (Table 2; P < 0.001). However, the equivalent sound levels are regularly higher than 85 dB(A). The patients would be exposed to equivalent sound levels of <85 dB(A) in 86.6% of cases with 49.6% exposed to a maximum between 75 and 79 dB(A), and in 13.4% of cases exposed to 85–100 dB(A).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The risk of hearing damage due to noise has been known since Plinius reported in 50 B.C. that people living near the rapids on the river Nile showed hearing impairment (Owen, 1995). In addition, a correlation between hearing loss of pilots and the number of hours flown was proved in the 1930s (Dickson et al., 1939). However, there were no data on the risk of persons working in alpine helicopter rescue organizations. With special regard to noise exposure, alpine rescue operations are characterized by at least two situations which differ from any other type of helicopter operation. The first is winch operations. Here the rescuer as well as the winch operator are directly exposed to the noise of the engine, which is only 1–1.5 m from their ears. Because the aircraft door must stay open during the whole procedure the personnel inside the aircraft are exposed as well. The second is so-called ‘hot-loading’, which describes a situation where a helicopter hovers above inclined ground and the personnel load the patient and/or material with the engine running at full power within only 1–3 m distance.
Although helicopter noise is only about 1/10 000 of the whole aircraft’s power, it is a source of noise of enormous intensity (Kloppel et al., 1993). This noise consists of several components: (i) periodic ‘rotational noise’ of low frequency; (ii) stochastic ‘vortex noise’ with frequencies >200 Hz and a spectrum more continuous than that of rotational noise; (iii) some constant peaks at high frequencies. The latter are caused by the transmission, turbine and other parts (Heinig, 1971; Laudien, 1976). In summary, helicopter noise is characterized by a broad spectrum of frequencies, impulses and tonality.
The noise we measured inside the cabins of the Alouette III and Alouette II ‘Lama’ agree with the results obtained in other older helicopter types (survey in Table 3). Differences are caused by differences in construction as well as individual differences between helicopters of the same type (Wolf et al., 1988). Our data measured outside the aircraft are also in agreement with the results of other authors, e.g. Lorenz and Demus (1965), who reported 116–121 dB(A) near a Mi4. However, in general there are few data which are comparable with ours, mainly because most authors have investigated helicopters during low level flight to obtain information about environmental noise exposure.
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There is an international consensus that noise exposure >85 dB(A) equivalent sound level is a potential risk. It causes aural and extra-aural effects which can be temporary or permanent, depending of the sound level and the duration of exposure. The excessive noise during alpine rescue operations is an additional risk, e.g. by disturbing communication. The following example illustrates the potential danger of this problem. During a rescue operation near Simplon Pass during bad weather one of the authors (T.K.) was given the order to jump down from the helicopter’s skid
3–4 m because any winch operation would have been too time-consuming under the conditions with the risk that the aircraft could be blocked in the gorge by fog. To do so the author had to remove his intercom gear and step outside onto the skid during the approach. When he got a sign from the pilot he jumped. Unfortunately, this was a misinterpretation of the pilot’s gestures and the pilot could not balance the loss of 100 kg (person plus equipment) on one side of the centre of gravity. The rotor touched the ground, which fortunately was snow, otherwise a crash with minimal chances of survival of the whole crew would have finished the manoeuvre. On any day with rescue operations the recommended limits of exposure calculated for a normal working day of 8 h were exceeded. Communication by intercom caused a further increase in noise exposure by +3–6 dB(A) (Glen and Moorse, 1977; Wolf et al., 1988; Owen, 1995). The possibility of reducing noise are limited due to the procedures during helicopter rescue operations as well as for technical reasons. Active noise systems which reduce the sound level in aeroplanes by about –10 to –20 dB(A) (Niesl and Arnaud, 1996) cannot be used in helicopters because these systems cannot cope with the intensive tonal component of the transmission (Niesl and Arnaud, 1996). Therefore, passive systems with an integrated intercom device are normally used inside helicopters. At the bases under study these systems were Peltor Aviation Headset 7003 (noise reduction rate 24 dB), Telex Hearing Defenders DBM-950 and DBM-1000 (noise reduction rate 25 dB) and David Clarc H10-50 (noise reduction rate 27 dB). These systems are all, according to FAA-TSO C57, C58, Cat B, adequate for noise protection inside the aircraft investigated if the crew use them regularly. However, during winch operations as well as any other work to be done outside the helicopter while the engines are working, the protection given by these systems is limited.
Other rescue procedures limit noise protection as well: 2% of all missions are performed by ‘hot loading’ (T. Küpper, unpublished data). The duration of hot loading is on average 3 min, which means that the recommended limits of exposure are exceeded 25-fold during this manoeuvre. Here, as well as during winch operations, there is a special technical problem: up to now no helmet has been available which can be used in an alpine environment according to DIN EN 12492 ‘Climbing Helmets’ (NN, 2000) and also for aviation and which is able to realise an adequate noise reduction of 30 dB(A). This is an option for further technical development. The optimal helmet should combine the following characteristics: protection against high energy impacts (aviation accidents), protection against high speed impacts (e.g. rock falls), noise protection of at least 30 dB, good ventilation (heat production during work at the site of the accident), an intercom device (stationary to be used inside the helicopter and mobile to be used at the site of the accident to provide constant contact with the helicopter) and undisturbed communication with persons at the site of the accident. Integrated sunglasses with high UV protection (e.g. a visor) and the possibility to fix a light would improve the comfort of such a helmet.
A comparison of Tables 1 and 2 demonstrates that the noise-limiting effect of more advanced helicopters, like the Bk 117 or the even quieter EC 135, is limited, mainly because of the extreme noise levels during rescue procedures outside the aircraft. Although significantly less noisier, the equivalent noise levels are still much higher than 85 dB(A) and therefore the use of a noise protection device is necessary here as well.
In contrast to most other occupational noise exposures helicopter rescue causes longer periods with daily high noise impact (‘season’) and periods with very little or no exposure (‘off season’, typically in November). In the literature there are no data available that investigate the risk of hearing damage caused by such a pattern of exposure. All risk estimations are based on statistical data of the energy equivalent noise level for 5 days per week and 2 days off (weekend). Any irreparable threshold shift (permanent threshold shift, PTT) is assumed to be the consequence of a prolonged energy deficiency of the cochlear cells. This causes damage to the structural integrity of the cells. Any temporary energetic deficiency of the cells caused by noise exposure can be measured as temporary threshold shift (TTS, ‘exhaustion threshold’). This status requires a pause in noise exposure until complete recovery, otherwise permanent damage will arise. However, in a situation like helicopter rescue with longer periods of exposure and delayed pauses it must be assumed from the viewpoint of energetic deficiency that there is a risk of PTT (ear damage). Here intelligent job rotation could be an efficient procedure for noise reduction, but this is only possible if enough educated crew members are available. At most bases investigated, more than one crew is available, normally one on the base and the other on call if further personnel should be needed. Both crews rotate with the other. However, this is only true of the pilots, rescuers (non-physicians) and winch operators, not of the physicians. At most bases only one physician is available as a crew member for all rescue operations and is therefore exposed to the full spectrum of noise exposure. At bases which perform rescue operations as well as commercial flights (transport, sightseeing, heliskiing, etc.), other crew members have a higher impact than investigated in our study due to these activities (e.g. Zermatt and Raron). In consequence, intelligent job rotation could be a strategy for noise reduction for the crews, but in reality this is limited by other activities, as described above, or by loss of personnel due to sick leave, holidays or other causes.
The amount of hearing loss of crews working in alpine helicopter rescue systems has not yet been investigated systematically. Pasic and Poulton (1985) stated that there should be no risk at all. However, they did not investigate alpine rescue but rather interhospital transfers. In consequence, there were no situations with crew working outside the aircraft with the engines running and they investigated a quieter type of helicopter (Bell 206L Jetranger II). Investigations of military helicopter crews showed that total flying hours were linked to an increased risk of hearing impairment (Peters and Ford, 1977; Edington and Oelmann, 1982; Ribak et al., 1985; Fitzpatrick, 1988). However, a comparison of the different investigations is difficult because of the numerous confounding factors (Owen, 1995). Like all the other investigators, we have regarded the time between flights as being quiet, without risk of hearing loss, but this is not realistic, as Matschke and others points out [concerts, portable stereo use, etc. (Babisch et al., 1988; Ising et al., 1988; Krähenbühl et al., 1988; Esser, 1992; Matschke, 1993)].
In summary, we conclude that the personnel of alpine helicopter rescue operations are at risk of noise-induced hearing loss and need adequate noise protection as well as examination and follow-up by occupational medicine professionals, e.g. according to the German regulations ‘G20 Noise’ (NN, 1989). On any helicopter base and during many rescue operations crew members can be found temporarily or constantly without noise protection. From the viewpoint of preventive medicine this is not acceptable. Protection devices should be used uninterrupted while the engines are running, especially during work outside the aircraft. In contrast to the personnel, the patient’s risk of a permanent threshold shift is extremely low, but the patient should be given a noise protection device in order to prevent temporary threshold shifts and to make the difficult and sometimes unstable situation more comfortable. Many reactions of the autonomous nervous system to noise of high intensity which may worsen the patient’s situation have been well-known for decades, e.g. heart frequency and blood pressure, and have been surveyed by Jansen (1981) and Jansen et al. (1996). Until the patient is given a protection device by the crew he should cover his ears with his hands while the engines are running (if he is able to do so). During winch operations the rescuer should take care to place the head of the patient, who is bedded in a rescue bag in a horizontal position, under the helicopter’s chassis to reduce the exposure by 6 dB(A). This must be done before the aircraft starts moving, as otherwise the bag will be stabilized by the airstream.
Acknowledgements—The authors would like to acknowledge Mr Theo Peters and Mr Siegfried Hofbauer (both State Institute for Occupational Health and Safety of North Rhine-Westphalia, Duesseldorf) and Mr Lutz Richter (Department for Occupational Medicine, Heinrich Heine University, Düsseldorf) for their assistance in data acquisition and evaluation. We would also like to thank Air Zermatt AG (Zermatt and Raron) and the air rescue bases at Innsbruck and Landeck (Prof. Dr Flora) for their assistance.
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FOOTNOTES |
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* Author to whom correspondence should be addressed: Am Botanischen Garten 15, D-40225 Düsseldorf, Germany. Tel: +49-211-9042935; fax: +49-211-9042996; e-mail: tkuepper{at}ukaachen.de
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Babisch W, Ising H, Dziombowski D. (1988) Einfluß von Diskothekenbesuchen und Musikgewohnheiten auf die Hörfähigkeit von Jugendlichen [Influence of music events on the hearing ability of young people]. Z Lärmbekämpf; 35: 1–9.
Dickson EDD, Ewing AWG, Littler TS. (1939) The effect of aeroplane noise on the auditory acuity of aviators. J Laryngol Otol; 54: 531–43.
Edington K, Oelmann BJ. (1982) An audiometric survey of army aircrew. Middle Wallop, UK: HQ Director Army Air Corps.
Esser L. (1992) Da geht die Post ab-Lärmbelastung Jugendlicher in Beruf und Freizeit [Noise exposure of young people at work and during leisure time]. Hörakustik; 27: 4–14.
Fitzpatrick DT. (1988) An analysis of noise-induced hearing loss in army helicopter pilots. Aviat Space Environ Med; 59: 937–41.[Medline]
Glen MC, Moorse SA. (1977) The contribution of communication signals to noise exposure. London: HMSO.
Heinig K. (1971) The periodic component of the rotor and propeller sound. Aachen, Germany: Messerschmitt-Bölkow-Blohm.
House JW. (1975) Effects of helicopter noise on pilot’s hearing. Trans Pacif Coast Otoophthalmol; 56: 175–86.
Ising H, Babisch W, Gandert J, Scheiermann B. (1988) Hörschäden bei jugendlichen Berufsanfängern aufgrund von Freizeitlärm und Musik [Permanent threshold shift of young people induced by noise during leisure time]. Z Lärmbekämpf; 35: 35–41.
Jansen G. (1981) Phaenomen Lärm und Arbeitsmedizin: extraaurale Lärmwirkungen [Phenomenon noise and occupational medicine: extraaural noise effects]. Diagnostik; 14: 552–9.
Jansen G, Schwarze S, Nothbohm G. (1996) Lärmbedingte Gesundheitsstörungen unter besonderer Berücksichtigung extraauraler Lärmwirkungen [Noise-induced health problems with special regard to extra-aural noise effects]. Z Lärmbekämpf; 43: 31–40.
Kloppel V, Lowson MV, Fiddes SP, Bernardis E, Francescantonio P, Campos LMBC, Macedo CM. (1993) Theoretical studies undertaken during the Helinoise Programme. 19th European Rotorcraft Forum. Cernobbio/It, 14.-16.9.1993.
Koch J, Koch C. (1990) Schwingungsbelastungen: Das geeignete Rettungsmittel [Vibrations–the adaequate patient transport system]. Notfallmedizin; 16: 491–5.
Krähenbühl D, Arnold W, Fried R, Chüden H. (1988) Hörschäden durch Walkman? [Permanent threshold shift induced by walkman?]. Laryng Rhinol Otol; 66: 286–9.
Laudien E. (1976) Main and tail rotor interaction noise during hover and low-speed conditions, paper presented at the 2nd European Rotorcraft and Powered Lift Aircraft Forum, Bückeburg, Germany.
Lorenz W, Demus HG. (1965) Lärmprobleme beim Hubschrauberkrankentransport [Noise problems during patient transport by helicopter]. Verk Med; 12: 525–32.
Matschke RG. (1993) Gehörschäden durch nichtberuflichen Lärm [Hearing loss induced by non-occupational noise]. Dt Ärztebl; 90 (34/35): A1 2240–2242.
NN. (1989) Berufsgenossenschaftliche Grundsätze für arbeitsmedizinische Vorsorgeuntersuchungen, G20 ‘Lärm’. [Procedure for the examination ‘noise, G20’ in occupational medicine]. St Augustin: Hauptverband der Gewerblichen Berufsgenossenschaften.
NN. (1994) DIN/EN 60651; IEC 651: Schallpegelmesser. [Noise measure equipment]. Berlin: Beuth Verlag.
NN. (1997) DIN 45645-2 Ermittlung von Beurteilungspegeln aus Messungen [Calculation of equivalent noise levels from measured data]. Berlin: Beuth Verlag.
NN. (2000) DIN EN 12492 Bergsteigerausrüstung: Bergsteigerhelme—Sicherheitstechnische Anforderungen und Prüfverfahren [Equipment for alpinists—safety requirements and testing procedures]. Berlin: Beuth Verlag.
Niesl G, Arnaud G. (1996) Low noise design of the EC 135 helicopter, paper presented at the 52nd Annual Forum of the American Helicopter Society. Washington, DC: American Helicopter Society.
Niesl G, Laudien E. (1994) Helicopter internal noise, paper presented at the 78th Meeting of the AGARD structural and material panel. Lillehammer: AGARD report no. R-798.
Owen JP. (1995) Noise induced hearing loss in military helicopter aircrew–a review of the evidence. J R Army Med Corps; 141: 98–101.[Medline]
Pasic TB, Poulton TJ. (1985) The hospital-based helicopter—a threat to hearing? Arch Otolaryngol; 111: 507–8.
Peters LJ, Ford, H. (1977) Extent of hearing loss among army aviators at Fort Rucker, Alabama. London, HMSO.
Ribak J, Hornung S, Kark J, Froom J, Wolfstein A, Askenazi E. (1985) The association of age, flying time and aircraft type with hearing loss in the Israeli Airforce. Aviat Space Environ Med; 56: 322–7.[Medline]
Rood GM, Glen MC. (1977) A survey of noise doses received by military aircrew. London, HMSO.
Wolf C, Ebm W, Chichini G. (1988) Die Lärmbelastung von Fluglehrern und Privatpiloten [Noise exposure of private pilots and instructors]. Arbeitsmed Sozialmed Präventivmed; 23: 88–90.
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