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 Leq8h) 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  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 . 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 . 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  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) . Another solution could be so-called in-ears earplugs, which today are used primarily by musicians . 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  despite our noise measurements in general being lower than reported (>85 dB) . 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 Leq8h 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 . 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 . 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 was carried out on four different physician manned units in a Danish prehospital setting. Both the noise exposure recordings and DPOAE measurements were done on physicians only which is a limitation to our study. The noise recorded during MECU emergency responses is believed to reflect actual noise exposure during these types of emergency responses and is comparable at least on a national basis since similar cars and sirens used nationwide. The ambulance recordings were done only when the patient was escorted by the physician and therefore reflects noise exposure in the treatment compartment during the use of siren. It is possible that the noise exposure in the driver’s cabin is different than those recorded in the treatment compartment however; other personnel working in the treatment compartment during the use of sirens will most likely be exposed to similar noise levels as the physician. Secondly, different types of ambulances are used in Denmark which makes a direct translation of the results difficult. Worth noting, we did not find any significant differences in the recordings though several different types of ambulances were included. The HEMS recordings were done both for pilots and physicians in a way that accurately reflects the actual noise level. To generalize these results to other services require that the same helmets and earmuffs are used. In general we recognize that noise results like these can be difficult to generalize to different prehospital settings, however we believe they will help increase focus on noise being a potential occupational hazard in the prehospital emergency medical settings. Moreover it has previously been speculated that the position of microphones close to the body can cause reflection artifacts causing errors in measurements . However, the method has been validated to be as accurate as using a dosimeter placed according to ISO 9612 with the generally accepted 2 dB measurement uncertainty . Another potential weakness of our study is the possibility of conduction of noise through the body (bone conduction) because of vibrations (especially in the HEMS) since the measurements in this study reflect the air-conducted noise only. As with every study, the risk of recruitment bias is present; i.e. if a person with a known hearing loss rejects enrollment to the project, the possibility of detecting an overall hearing loss is less if all subjects are known to have a good hearing.