The typical NIHL audiogram shows a distinct high tone hearing loss centered at 4,000 or 6,000 Hz (Figure 1 ), often referred to as a “noise notch,” while hearing at 1,000 and 2,000 Hz is largely unaffected. Hearing at 3,000 Hz should be close to the average of 2,000 and 4,000 Hz (3 , 4 ). NIHL will always display this pattern, but other types of hearing loss, especially hereditary types, can have the same shape. In NIHL, hearing in the two ears will be symmetrical (5 ), unless a specific event, such as an explosion near one ear, can account for a side difference. The development of NIHL follows a characteristic course, shown graphically by Taylor et al. (6 ) and in tabular form by ISO-1999 (2 ): during the first ten years of exposure, there is a rapid deterioration at 4,000 Hz and a parallel, but milder one, at 3,000 Hz. The deterioration at both frequencies saturates after ten years so; after that time, little progression occurs despite continued exposure. In contrast, at 2,000, and to some extent at 1,000 Hz, a linear loss develops gradually and slowly. Accordingly, a 4,000 or 6,000 Hz loss that progresses beyond ten years and often reaching 80 - 90 dB should be suspected to have causes other than noise.
The Expected Audiogram of a 55-Year-Old Man After 30 Years in Noise at 95 dBA
Predicted from ISO-1999.
Therefore, an asymmetry between the two ears, a hearing loss at a single frequency below or above 4,000 - 6,000 Hz, a marked deterioration starting at 1,000 or 2,000 Hz, or a low and/or middle frequency loss indicates that factors other than noise have influenced the audiogram. However, such factors often act in combination with noise, so that the resulting audiogram becomes a combination of NIHL and other causes.
When there is asymmetry, a tumor of the acoustic nerve (acoustic neuroma) must always be ruled out. For this reason, our criterion for asymmetry was adopted from an acoustic neuroma protocol (7 ). A hearing loss in the low/mid frequency range (250 - 2,000 Hz) cannot be NIHL and rather represents middle ear disease, otosclerosis, Meniere’s disease, or hereditary conditions. High tone hearing losses with configurations that are not consistent with NIHL are usually of a hereditary nature. The so-called ski-slope audiograms (Figure 2 ), which are often confused with NIHL, represent several types of progressive hereditary hearing loss characterized by a high frequency loss that is conspicuously wider and more rounded than a “noise notch” (3 ). At an early stage it may be fully identical to NIHL, but as it progresses, deterioration at 2,000 and 3,000 Hz will bring about the typical appearance. Finally, it must be kept in mind that the normal age-dependent deterioration of hearing also manifests itself in the high frequencies. Tables of normal hearing at different ages can be found in ISO-1999 (2 ).
Typical Ski Slope Audiogram
Actual audiogram of 65-year-old man with only minimal noise exposure. Note the rounded shape of the hearing loss between 2,000 and 8,000 Hz and the pronounced loss at both 2,000 and 3,000 Hz.
The findings of this study are, in all probability, fairly representative of present-day conditions in Danish industry, and therefore it is of interest to consider how our data might figure in the WEA statistics.
It is often the case that a person’s hearing loss is reported to the WEA as suspected NIHL simply if the person has been noise exposed. Therefore, the 159 persons listed in Columns B, C and D who did not have normal hearing could easily have entered the statistics. In that case, the WEA could reject the 43 cases in Column D because the configuration of the audiograms would argue against NIHL, but it is, nevertheless, conceivable that some of the 16 ski slopes would be acknowledged. A substantial number, but not all, of the 88 in Column B would be accepted, but in the individual case it would depend on a detailed examination of all relevant information and documentation. Acknowledgement requires a noise exposure of at least five years in 85 dBA, an audiogram consistent with the exposure, and no evidence of a more likely cause of the hearing loss. Several of the 28 persons in Column C definitely would not satisfy these criteria, but it is impossible to say how many. Accordingly, it is evident that far from all 116 cases of probable or possible NIHL (Columns B and C) of the 159 reported (73 %) would be acknowledged, but, at any rate, the figure would be higher than 41 % of the NBII statistics. The two figures do not necessarily disagree, since the persons in our study were drawn from a very noisy industry that would necessitate expecting a high NIHL prevalence.
Acknowledgement of NIHL only qualifies for compensation if the disability ascribed to NIHL, as mentioned, amounts to at least 5%. In Denmark, NIHL disability is set by the procedure devised by Salomon et al. (8 ), which defines 5% auditory disability as the condition where a person cannot manage a two-person conversation in background noise without lip reading. In the individual case, this is determined by otolaryngologists who specialize in audiology using word recognition measurements in quiet and in background noise. We have analyzed 350 such affidavits, and on this background we estimate that only 11 of the 116 persons (9%) in Columns B and C would attain 5% disability. In addition, several of them would not have occupational NIHL, but rather hearing loss caused by leisure noise or hereditary conditions.
Seventy-one (Columns C and D) of the 159 who failed the screening (45%) had a hearing loss that was definitely influenced or caused by factors other than noise. This agrees reasonably with the 53% reported in a comparable study (9 ), indicating that about 50% of those who fail an industrial hearing screening do not have NIHL. When the purpose is to detect NIHL, such a high rate of incidental findings is undesirable, but since they sometimes reflect serious conditions that require intervention, detecting them may be at least as important as detecting NIHL, as far as the individual is concerned. Nevertheless, the high rate of incidental findings has two important general implications for industrial hearing screening: it erodes the efficiency of the screenings designed to monitor NIHL occurrences, and it makes it imperative that those who fail be seen subsequently by an otolaryngologist.
Since the relationship between noise exposure (intensity and duration) and NIHL is a well-established fact, it is interesting that our study failed to demonstrate such a correlation. There may be several explanations for this: one is that many of the hearing losses in our study had causes other than noise, at least exclusively. Another is that exposure time itself does not imply that the noise level has been harmful, so many persons, despite long exposure, may not have been at risk of NIHL. Since in about 50% of the cases the exposure had begun in the late “nineties,” when the preventive measures were becoming effective, this group may be large enough to mask the effect of exposure time in individuals who had been exposed to harmful noise levels. This may also explain the age differences between individuals with and without a hearing loss entirely in the high frequencies: these persons were old enough to have acquired their NIHL at a time when the noise problem in the work environment was severe.
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