In recent years, the topic of occupational noise exposure has received increased attention. The risk of permanent hearing loss due to work-related noise exposure is a well-known issue with an estimated 22 million U.S. workers exposed to hazardous noise each year [1]. Moreover, 34% of those exposed to hazardous noise report no use of ear-protective gear [1]. As stated in the E.U. Directive 2003/10/EC for Noise Exposure, employees should not be exposed to more than 87 dB(A) on a daily basis (during 8 h) [2]. In this directive, it is recommended that hearing protective gear should be available to employees exposed to noise levels of 80 dB(A), and it is stated that its use should be mandatory at values exceeding 85 dB(A) [2]. It is well known that emergency responses in themselves pose a risk to the pre-hospital personnel in the form of traffic accidents [3]. Another occupational risk that in particular may be relevant to emergency medical technicians (EMTs) and pre-hospital physicians working in Mobile Emergency Care Units (MECU) or Helicopter Emergency Medical Services (HEMS) is noise exposure. Physicians and EMTs may not be fully aware of the intensities of noise to which they are exposed during the day, with a large range of noisy tasks performed in the pre-hospital environment. Thus, they may not be aware of the possible consequences of this exposure on their hearing ability.

A few studies have evaluated patient and crew noise exposure during ambulance transport both in ground-based units, in fixed-wing operations (medical aeroplanes) and in alpine rescue helicopters with contradictive findings [4–7]. According to one study, transportation by fixed-wing airplane does not affect hearing with noise exposure levels reported just below the limit for acceptable occupational noise exposure (85 dB(A)). However, in that particular study, no hearing tests were carried out, and the question of possible hearing loss was thus unresolved [4]. Reducing noise exposure with protective gear (up to 10 dB) to the flight crew is already a priority and contributes to an acceptable equivalent noise level below the 85 dB(A) threshold [4]. However, noise exposure during alpine helicopter rescue operations seems to exceed the national occupational levels with exposures equivalent to noise levels of >85 dB(A) recorded during 2726 missions from four bases [7]. The setting during an alpine rescue differs from other Helicopter Emergency Medical Services by often “hot-loading” the patient (the patient is loaded while the engine is on). This principle, however, is also used on occasion by non-alpine HEMS rendering the ensuing noise exposure relevant to these organisations. Protective gear is already used but may be insufficient in protecting the crew from loss of hearing [7], especially when working outside of the helicopter. A study in 521 French military helicopter pilots who underwent pure-tone audiometry prior to their annual medical examination exhibited abnormal hearing especially in the high frequencies at 3 kHz and 6 kHz [8]. Moreover, fighter- and helicopter pilots seemed more vulnerable to hearing loss than transport pilots [8]. Thus, a risk of work-related hearing has been established in the flying services.

Emergency medical technicians (EMTs) operating in ground ambulances seem to be exposed to high levels of noise [5, 6]. The highest recorded sound levels have been measured with sirens engaged en route during live emergency responses. One study showed that when the sirens were on, all noise values measured inside the cabin exceeded the national occupational health regulation of 85 dB(A) as a mean of 96 dB(A) was measured [5]. The average siren noise exposure during a work day of 8 h was 16.22 min (range 5–33 min), reaching 20.33 min on busy days (range 7–42 min) [5]. These measurements were only conducted in four ambulances under three different circumstances (12 recordings in total) [5]. Based on 31 noise recordings, a second study found noise levels up to 84 dB(A) in the front passenger seat [6]. The two studies concluded that the occupational noise exposure resulted in excessive hearing loss among EMTs especially in the high frequencies, which typically are the frequencies involved in noise-induced hearing loss [5, 6]. Additionally, they reported a noise level ranging between 93 and 106 dB(A) inside the cabin during the use of sirens [5] which, according to E.U. directive, would require the use of ear-protective gear as the noise level exceeds 85 dB(A) [2]. Excessive noise exposure may lead to a temporary threshold shift (TTS) in hearing. This TTS is normally quantified with audiometry but the level shift of amplitudes measured with Distortion Product Otoacoustic Emissions (DPOAE) can be used as a measure of altered sensory outer hair cell (OHC) function in the inner ear which can be an indicator of affected hearing [9]. In that study, a temporary shift in pure-tone audiometric thresholds and a reduction of the DPOAE amplitudes were documented within the frequency range 1–6 kHz following exposure to excessive noise at sporting events [9]. The measurement of sensory OHC function can be performed quickly with DPOAE measurements making it feasible in the pre-hospital environment for detecting impacts on hearing [9].

The aim of this study was to investigate the overall noise exposure during pre-hospital physicians’ pre-hospital work in the MECU and civilian HEMS and to clarify whether a change in amplitude levels of DPOAE occurs following noise exposure. Any association between noise exposure from MECU and HEMS and change in DPOAE amplitudes was investigated. Furthermore, in an attempt to reveal any occupationally related hearing loss, a group of EMTs, pre-hospital physicians and in-hospital controls underwent a standardised pure-tone audiometry.

The investigation resulted in a total of 444 sound measurements performed during emergency responses (corresponding to 222 emergency responses) with the three MECUs in the sampling period. Each MECU physician was measured one to 34 times in total. The sound measurements in the HEMS were performed on pilots and physicians only. Each HEMS physician or pilot was measured 5–44 times, and the investigation resulted in a total of 414 sound exposure measurements. Finally, each member of the in-hospital control group was measured 1-4 times. The in-hospital investigations resulted in a total of 66 sound exposure measurements (33 for each ear). Furthermore, 408 post-exposure DPOAE measurements in total for the MECU physicians and 138 for the HEMS personnel (pilots and physicians) were obtained. Identical measurements were performed in in-hospital controls every two hours, in total collecting 32 DPOAE measurements. Finally, 12 pre-hospital physicians, 39 EMTs and 10 in-hospital controls underwent a standardized pure-tone audiometry test.

Table 1 also lists the measured range of peak values during the different types of exposure. The largest noise exposure levels were obtained during HEMS operations with peak values reaching up to 128 dB(C) for both pilots and physicians. Again, a linear correlation is seen in the MECU group with increasing peak values as the speed of the vehicle increased.

Figure 1 shows the coefficients obtained from the linear mixed-effects regression model. All coefficients were analyzed by comparison with a reference situation, defined as the exposure of physicians working in an in-hospital environment. The linear mixed-effects regression model uses all obtained measurements and predicts the most probable noise exposure for the different exposure situations (such as MECU on rural road, MECU on motorway etc.). The exposure-specific coefficients in Fig. 1 obtained from the statistical analysis with the linear mixed-effects regression model were shown to be very similar to the mean values of the measurements in Table 1. The different types of exposure were all significantly larger than the reference coefficient (in-hospital controls). Especially interesting is the finding of the coefficients for HEMS physicians and pilots flying with earplugs, which is shown to be 11 dB(A) larger than personnel flying without standard earplugs (single-use foam or custom-made earplugs). A comparison of the noise exposure for the three MECUs demonstrated no significant difference between the three vehicles. Neither did we find any differences in the ambulance recordings.

The primary findings of this study indicate that the overall pre-hospital noise exposure during single emergency responses with MECUs and ambulances exceeds the threshold defined in the E.U. Regulative for Noise (>80 dB(A)). In single flight missions, the noise exposure with the HEMS also exceeds the threshold with exposures reaching up to 90 dB(A). However, corrected for the collective noise exposure during a whole working day, the average exposure is expected to be below the threshold for daily noise exposure. Physicians in MECU and HEMS were on average exposed to (of L_eq8h) 71 dB(A) and 73 dB(A), respectively, and 73 dB(A) in pilots. The low average noise exposure may be the main reason why no evidence of occupational hearing loss with pure-tone audiometry could be found. In a small sample, a level change of DPOAE amplitudes for HEMS personnel at 4 kHz indicated, however, that sensory OHC function was affected and this may have a potential impact on hearing thresholds if this exposure is frequent enough.

To our knowledge, this is the first large study with systematical and simultaneous measurements of left and right ear noise exposures to pre-hospital physicians in the MECU and civilian HEMS. The largest previous study [6] only included 31 measurements in ambulances compared to our 444 MECU and ambulance recordings during different tasks (emergency response on urban roads, rural roads or motorways). Furthermore, our method using personal noise recording microphones worn just outside the auditory canal was an exact exposure model which is comparable to dosimeter use. As stated, helicopter noise exposure during alpine rescue missions has been thoroughly investigated. However, our new approach with microphones worn underneath the ear muffs in the protective helmets gives a more precise insight into the noise actually reaching the ear instead of measurements in the helicopter cabin [7]. The recorded values of 79 to 80 dB(A) in ambulances are partly in agreement with previous studies (84–96 dB(A)) with a slightly lower overall noise exposure during the use of sirens [5, 6]. A possible explanation for this is an increased focus on work-related environmental noise exposure resulting in a more appropriate siren placement and better acoustic insulation in ambulances. Despite these efforts, with maximum values reaching 94 dB(A) during ambulance transport on rural roads, noise exposure still exceeds acceptable levels and, according to the occupational guidelines, ear-protective gear must be present in the ambulances. According the E.U. regulation, these recommendations apply even if average noise exposures are below acceptable noise limits for the whole working day. The use of ear-protective gear in ambulances might impair communication with the patient during the transport to the hospital. One possible solution to this problem could be to install intercom systems to be used during transport by physicians, EMTs and the patient or to further improve the acoustic insulation of the ambulances. The noise exposure measurements in the MECU on average do not exceed the occupational noise exposure limits with measured values ranging from 72 to 77 dB(A). Semi-high levels of noise are generated when driving on motorways, probably because of the increased speed, increasing the amount of wind-generated noise. HEMS personnel not using earplugs underneath the earmuffs are exposed to average noise levels up to 81 dB(A) - and 92 dB(A) if earplugs were used.

This difference is likely caused by an impaired ability to hear the intercom when using earplugs, causing HEMS personnel to turn up the volume. Often, single-use foam earplugs were used instead of custom-made ones. Single-use foam earplugs can be expected to attenuate noise with approximately 10–20 dB(A) or less, because of variable fit to the individual auditory canal [14]. As a consequence, the protective effect of single-use foam earplugs is levelled out by increased intercom volume. Non-earplug users are still exposed above the >80 dB(A) threshold for acceptable noise according to the European Occupational Regulative [2] despite helmet usage. Therefore, further steps should be taken to reduce the occupational noise exposure to HEMS personnel. A possible intervention could be implementation of active noise reduction in all helmets. This is a well-tested initiative that for fighter pilots with similar exposure can reduce noise by 8 dB(A) [15]. Another solution could be so-called in-ears earplugs, which today are used primarily by musicians [16]. In-ears are custom-made earplugs with the possibility of radio-connection directly, and they can be used underneath the earmuffs. The advantage of in-ear plugs is that a direct connection to the intercom is possible. With the intercom inserted in the auditory canal, low volume levels can be used for communication. Overall, our findings are in accordance with earlier studies [7] despite our noise measurements in general being lower than reported (>85 dB) [7]. This difference is likely caused by the differences in methods. DPOAE post-exposure measurements were only affected for the HEMS group (Table 3) at 4 kHz when compared to MECU physicians. For both MECU and HEMS personnel, it is important to notice that the actual exposure time is limited to 42 and 41–44 min, respectively, during 8 or 12 h of recording. This limits the average daily noise exposure to L_eq8h of 71 dB(A) for MECU and 73 dB(A) for HEMS physicians. Both groups normally have shifts that last up to 24 h. For 24-h shifts for MECU, the average exposure will be increased by 4.8 dB compared to 8-h shifts and the increase for HEMS is 3 dB if compared to the 12-h shifts. The large distribution in 95% CI for emergency responses in ambulance on motorway is due to the geographical location of the three MECUs. Especially the MECU in Kolding will have to escort a number of patients to a larger hospital (Odense University Hospital) with up to 45 min of response time on motorway. The exposure on a particular working day therefore depends on the type of exposure during that particular day.

The DPOAEs exceeding 7 dB S/N at baseline for analysis in the HEMS group at 4 kHz Hz is quite small (N = 19) and the result might therefore just be an incidental finding. There are limitations to the DPOAE measurements; a person with impaired hearing will have affected emissions at baseline which may or may not exceed 7 dB S/N. Therefore, an additional affection of the DPOAE signal due to noise exposure may decrease the amplitude further, resulting in DPOAE amplitudes not exceeding 7 dB S/N. Secondly, the DPOAE equipment is susceptible to surrounding noise, which is unavoidable in a pre-hospital setting.

The pure-tone audiometry test revealed no significant differences between the three groups. This might be because of our cohort size, as a previous study of a group of EMTs revealed an occupational hearing loss [5]. Reviewing all audiograms manually, no noise-induced hearing loss could be found in any of the 61 cases. Normally, a notch is seen in the 3–6 kHz area typically centered around the 4 kHz in noise-induced hearing loss [17]. In general, pre-hospital physicians attend the MECUs and HEMS between 3 and 6 times every month, while their remaining working hours is spent within the hospitals, where the noise exposure is significantly less. This little exposure to pre-hospital noise is not expected to affect the hearing and furthermore, the exact time exposed for sirens is low. This may be an explanation why no occupational hearing loss could be found in our small cross-sectional group (with an average of 10 active pre-hospital years). Besides the risk of an occupational hearing loss, the possibility of communication problems in both MECU and HEMS settings are present due to high noise exposure. This could affect the co-operation between pre-hospital physicians and EMTs.

Our study indicates that the overall pre-hospital noise exposure during emergency responses with MECUs, ambulances and HEMS missions exceeds the threshold defined in the E.U. Regulative for Noise (>80 dB(A)). Although no evidence of occupational hearing loss with pure-tone audiometry could be established and, in general, average noise exposure during the whole working day was low, we recommend either better insulation of MECUs and ambulances, or the opportunity to use ear-protective gear such as ear muffs or earplugs. Furthermore, ear-protective gear should likewise be offered to HEMS personnel as they are heavily exposed to noise despite the passive ear-protective gear already in use.