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Year : 2006  |  Volume : 8  |  Issue : 31  |  Page : 80-87
The risk of noise-induced hearing loss in the Danish workforce

1 Department of Occupational Medicine, Aarhus University Hospital, Denmark
2 Department of Audiology, Aarhus University Hospital, Denmark

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  Abstract 

The causal association between occupational noise exposure and permanent hearing loss is well-documented and well-founded primary preventive approaches have been developed. However, documentation of the impact on the present prevalence of noise-induced hearing loss in the working population is limited. This study reports on the prevalence of noise-induced hearing loss in a population sample of 788 workers from 11 trades with expected high noise exposure levels and a reference group examined according to the same protocol. Full-shift A-weighted equivalent sound levels were recorded and pure tone audiometric examinations were conducted at the work sites in soundproof booths. Data were analyzed with multivariate regression techniques and adjusted for age, sex, ear disease, smoking and environmental noise exposure. An overall two-fold increased risk of hearing handicap (hearing threshold above 20 dB averaged across 2, 3 and 4 kHz for either ear) was observed in the noise exposed workers [odds ratio (OR) 1.99, 95% confidence interval (CI) 0.91-4.34]. Workers exposed for more than 20 years to an exposure level above 85 dB(A) had a three-fold increased risk (OR 3.05, 95% CI 1.33-6.99). Workers starting in noisy work during the last 10-15 years or workers below 30 years of age showed no increased risk of hearing handicap. This indicates that preventive measures enforced during the past 10-15 years to reduce noise exposure may have borne fruit. Systematic surveillance of noise and hearing levels in appropriate populations should still be included in an efficient hearing conservation program.

Keywords: Health survey, hearing loss, mass screening, noise-induced, occupational diseases, risk assessment.

How to cite this article:
Rubak T, Kock SA, Koefoed-Nielsen B, Bonde JP, Kolstad HA. The risk of noise-induced hearing loss in the Danish workforce. Noise Health 2006;8:80-7

How to cite this URL:
Rubak T, Kock SA, Koefoed-Nielsen B, Bonde JP, Kolstad HA. The risk of noise-induced hearing loss in the Danish workforce. Noise Health [serial online] 2006 [cited 2019 Aug 19];8:80-7. Available from: http://www.noiseandhealth.org/text.asp?2006/8/31/80/33538

  Introduction Top


The causal association between occupational noise exposure and hearing loss is well documented and there is consensus about acceptable noise exposure levels. [1],[2],[3],[4] Recognized approaches to primary and secondary prevention have shown documented effect on rates of hearing loss in noise exposed worker populations and have been implemented by the regulatory agencies. [5],[6],[7],[8] Still, occupational noise induced hearing loss is among the most prevalent occupational diseases [9] and surveillance data documenting the preventive impact of targeted noise prevention remains sparse.

A proper surveillance strategy must monitor hearing loss, in addition to noise levels, since use of hearing protecting devices may be the only means to prevent occupational hearing loss, when the noise threshold levels are exceeded, but only if properly used. [7],[10]

We recently reported the noise exposure levels in a sample of 820 workers from 11 Danish industries. [11] Between 20 percent (manufacture of machinery) and 50 percent (construction) were exposed above the permissible exposure limit of 85 dB(A) averaged over an eight-hour working day.

In Sweden, improving hearing thresholds have been documented for noise exposed industrial workers during the 1970s, 1980s and 1990s. [12] In Denmark, the annual notification about occupational hearing loss to the workers' compensation board declined from about 3000 to 1700 of a total Danish population of five million between 1990 and 2004, while the reporting of occupational hearing loss has increased in Washington State. [13],[14] The number of notified cases of hearing loss does not provide valid information about the hazard as national statistics on work related hearing loss probably grossly underestimate the true occurrence. [15]

In this study we obtained an estimate of the risk of hearing loss in the Danish workforce to document the impact of the noise preventive measures. We exploited the fact that hearing thresholds are expected to show a steep linear increase during the first few years of exposure followed by a slowing of loss [1],[3] and focused our interest on the risk of young workers and workers entering a noise exposed job during the past few years.


  Materials and Methods Top


We identified the ten industrial trades, defined at the two-digit level of the Statistical Classification of Economic Activities in the European Community, NACE, [16] with the highest notification rates of noise-induced hearing loss according to the Danish occupational disease statistics. [14] We also included children day-care units because recent measurements in Denmark have indicated that employees of these units are exposed to full-shift equivalent sound levels about 80 decibel (dB). Office employees of financial companies and a sample of residents exposed to traffic noise were selected as the reference category.

The National Bureau of Statistics provided a complete list of 2047 companies within the 12 selected trades in Aarhus County by 1 August 2001. The survey was restricted to the 840 companies with 15 or more employees. Within each trade we randomly selected five companies with 15-49 employees and five with 50 or more employees. A number of companies were ineligible because of downsizing and less than 10 employees in the production area, transfer of manufacturing to other areas or closure. As companies declined to participate or turned out to be ineligible, the initial roster of 120 companies was consecutively updated with new randomly selected companies until five companies within each chosen trade and size-specific stratum had been included, no more companies were available or project resources had been exhausted (in that order).

Once a company had accepted participation, we asked for a complete list of the employees in its manufacturing departments since only workers in production areas were to be enrolled. Randomly selected employees on the roster aged 20-45 years were contacted by the project team at the worksite and invited to participate until a maximum of 10 participants had been reached. However, only six companies complied with the request to provide complete lists and in most companies foremen and managers selected the workers to have at least one employee from each work area. Where less than 10 satisfied the age criterion, we supplemented with older participants until the desired number was reached.

The group of residents was identified from addresses obtained through a survey in the Aarhus Municipality. The group was divided into 10 different geographical areas, so that different parts of the central town were included in the study. Two residents were working in industries with possibly high noise levels.

All study subjects worked or lived in Aarhus, the largest county in Denmark with a population of 650,000. The recruitment procedure has been described in detail elsewhere. [11] The scientific ethical committee of Aarhus County approved the study and study subjects gave written, informed consent.

Sound level measurement

The A-weighted equivalent sound level (L Aeq ) and the peak sound level were recorded every 5 seconds for 24 hours by portable dosimeters (Bruel and Kjaer, model 4443, Naerum, Denmark). Dosimeters were handed out at the work site and participants were instructed individually. The dosimeter was worn in a pouch attached to a belt at the participant's waist and the microphone was mounted at or near the participant's collar - on the right side if right-handed and the left side if left-handed. The dosimeters were set to a dynamic range of 50-120 dB(A). Because of technical problems calling the lower measurement range into question, a lower cut-off point at 58 dB(A) had to be used and measurements below 58 dB(A) were set to 0 dB(A). Peak levels were measured using a 10 -6 -second interval and the highest value per 5-second interval was recorded. Peak-values were measured using a C-weighted model with an exchange rate of 3 dB in the range 70-120 dB(A).

During sound level recordings, all participants kept a 24-hour diary to keep track of the timing of work, leisure and sleep hours. The noise exposure was computed for each of the respective time intervals as the L Aeq values. Since we obtained the full work shift recording, the L Aeq for work hours is equivalent to the time weighted eight hour value (L Aeq(8h) ).

Questionnaire information

A separate questionnaire was used by the participants to provide information on ear diseases (chronic otitis, Meniere's disease and rupture of the ear drum), work history, use of hearing protective devices (current use and total years) and noise emission during work and leisure time. Diaries and questionnaires were reviewed together with the participant to ensure complete and accurate information.

Noise exposure assessment

We expected sound levels to vary more from day to day for the individual workers than between different workers. [17],[18] We therefore classified the workers by the L Aeq -value calculated for the trade and not by his or her individual L Aeq -values. Residents were classified by the common L Aeq -value calculated for all residents. On the other hand, leisure-time or night-time noise exposure was classified by the individual values.

Time since first noise exposure was identical with duration of noise exposure since all study subjects were recruited during a one-year period and was defined as the duration of employment in the current job if the L Aeq -value calculated for the trade was above 80 dB(A). If the study subjects judged that any previous jobs had involved comparable or higher noise exposure levels, the total duration of occupational noise exposure was extended to embrace such job periods too. Residents and subjects in trades with L Aeq -values below 80 dB(A) were set to a duration of noise exposure of 0 years.

Pure tone hearing thresholds

Hearing thresholds were determined by pure tone audiometry at the workplaces in a mobile unit equipped with a soundproof booth (model AB-4240, Eckel Noise Control Technologies, Bagshot, UK). To avoid temporary threshold shift, all participants in the industrial trades were asked to use hearing protection from the beginning of the working day on the day of testing. Otoscopy was performed to verify that the ears were free of wax and that the tympanic membrane was imperforated. Pure-tone audiometry was performed at 0.25, 0.5, 1, 2, 3, 4, 6 and 8 kHz in 5 dB increments using a Madsen Voyager 522 audiometer equipped with TDH-39 headphones (Madsen Electronics, Taastrup, Denmark). The audiometer was calibrated every six months according to ISO standards. An experienced audiology technician conducted the tests.

Hearing handicap was defined as a hearing threshold averaged over 2, 3 and 4 kHz of at least 20 dB recorded for the right or the left ear and in accordance with one of several equations proposed in the ISO 1999 standard. [3] The prevalence of hearing handicap according to this definition was 11.5 percent in the reference population and is expected to represent the prevalence of hearing handicap among the noise-exposed workers if they had not been exposed to noise (the background risk). The prevalence reflects the age distribution, the hearing threshold measurements and other factors that determine the recorded hearing level in a population.

The 4 kHz frequency was included because this frequency was expected to be the earliest and most severely affected hearing frequency. [3] Other definitions of hearing handicap have been proposed, e.g., for worker compensation programmes. [19] This study, however, was not rooted in a wish to be applied by worker compensation purposes; it merely seeks to assess the burden of noise-induced hearing loss in selected trades and in groups of workers defined by levels and duration of noise exposure.

Statistical methods

We used logistic regression models to analyse the relative risk of hearing handicap (by the proc LOGISTIC SAS procedure). [20] The reference category was workers from the finance industry and the residents. All models included age (6 levels), sex and reported ear disease in addition to indicator variables for industry or noise exposure. We also controlled for smoking and leisure time and night time noise exposure but this did not affect the risk estimates and thus these variables were not included in the final analyses.

We analyzed the effect of time since first noise exposure for different levels of noise exposure on the hearing threshold level at 4 kHz, using linear regression modelling (by the proc GLM SAS procedure). [20] The distribution of hearing level was right-skewed and we log-transformed hearing levels prior to analysis. These models also included age, sex and reported ear disease. We assessed model fit by plots of residuals. Graphical displays of the relationships between noise exposure and geometric mean hearing threshold level were smoothed by spline regression techniques after exponentiation of the predicted values (proc GPLOT SAS procedure). [20]


  Results Top


We recruited 91 (66.9%) of the companies originally identified and eligible for the study. From these companies we enrolled a total of 785 workers during a one-year period (2001-2002). We also included 45 residents. The total study population comprised 830 subjects. The 788 subjects with a completed audiometric examination were included in the present analysis (95%). Thirty-five did not report a complete occupational history and analyses of duration of noise exposure were based on 753 subjects. [Table - 1] provides background characteristics of the study population of whom 684 were employed in noisy trades and 104 belonged to the reference population. The noisy trades employed more men, more smokers, more with a hearing handicap, and fewer subjects below 30 years compared with the reference category. The average leisure time noise levels were comparable for the two study populations and below 70 dB(A). The night time noise levels were higher in the noisy trades than the referents, but still far below the occupational levels.

[Table - 2] presents the noise exposure levels in the different noisy trades and the reference category (estimated as the L Aeq value for the trade). Highest levels were seen in companies manufacturing basic and fabricated metal (about 88dB(A)), day care workers experienced levels about 84 dB(A) while the referents on average were exposed about 76 dB(A). None of the equivalent sound levels computed on a trade level exceed 90 dB(A).

The risk of hearing handicap was increased for all the industrial trades but not for the day care units when compared with the reference category of financial company employees and residents after controlling for age, sex and reported ear disease [Table - 2]. The overall odds ratio (OR) for all noisy industries was 1.76 (95% confidence interval (CI) 0.81-3.80). The highest risk was seen for the construction industry with a four-fold increased OR for hearing handicap (OR 3.93, 95% CI 1.27-12.5) and construction of basic metals (OR 3.47, 95% CI 1.32-9.14).

When the subjects were classified by the L Aeq -value we observed an overall 74% increased risk of hearing handicap for those exposed at levels above 80 dB(A) compared with workers exposed at lower levels (OR, 1.74, 95% CI 0.80-3.76, [Table - 3]). Cross-classification of noise intensity level and time since first noise exposure revealed increasing trends both by noise level and time for workers exposed to noise for 10 years or more. Workers exposed for more than 20 years above 85 dB(A) had a three-fold increased risk of hearing handicap. On the other hand, workers exposed for less than 10 years, regardless of noise level, showed no increased risk of hearing handicap.

If this analysis was stratified by age, we observed no increased risk of hearing handicap among the young workers 20-29 years of age [Table - 4]. Only few of them experienced more than 10 years of noise exposure. Among the older workers, risk increased by intensity and time since first noise exposure.

The estimated age, sex and ear disease-adjusted mean hearing threshold at 4000 Hz rose by increasing time since first noise exposure [Figure - 1]. The trend was most significant for those experiencing the highest noise levels (≥85 dB(A)) and low for those exposed to the lowest levels (<80 dB(A)). This analysis suggested little hazardous effect of noise during the last 10-15 years of exposure.

Seventy-five percent of workers exposed to more than 85 dB(A) and 42% exposed to 80-84 dB(A) reported that they used hearing protection. Workers aged 20-29 did this more frequently and during a higher proportion of noise exposed working years than the older workers [Table - 5]. Duration of noise exposure was unrelated with hearing protection use (data not shown).

The subset of 183 workers reporting that they had used hearing protection for more than 75% of all years in a noisy trade showed no lower risk of hearing handicap than the workers with less frequent hearing protection use (data not shown).


  Discussion Top


Occupational noise exposure remains an environmental source of hearing impairment among workers. In this population sample of workers from trades with the expected highest noise exposure levels in Denmark, 30% had a hearing handicap, here defined as pure tone audiometric hearing thresholds above 20 dB averaged over 2, 3 and 4 kHz, which was twice as many as in the reference population. The risk was 74% higher among workers exposed to equivalent noise levels above 80 dB(A), corresponding to a noise excess fraction of 42% (EF=1.74-1/1.74), that is the fraction of cases that could be eliminated if noise levels were reduced. Workers exposed to noise for more than 20 years at an intensity above 85 dB(A) had a three-fold increased risk and two thirds of the cases of hearing handicap were due to occupational noise exposure [Table - 3].

We observed no increased risk of hearing handicap in workers younger than 30 years of age or among workers entering a noise exposed job during the past 10-15 years. This finding indicates a significant lowering of hazardous noise exposure during this period. This interpretation is premised upon an expected strong effect of noise on hearing thresholds during the first years of exposure that has been documented by several cross-sectional studies. [1],[2],[21],[22],[23] It should, however, be noticed that a recent three-year follow-up study of noise-exposed construction industry apprentices showed a less than expected increase in hearing threshold levels. [24]

Available data cannot clarify whether noise exposure levels have actually declined or if use of hearing protection has increased during the past 10-15 years. Our data indicated that young workers used hearing protection more consequently than the older workers.

Preschool teachers from day care units experienced unexpected, high L Aeq -values of about 84 dB(A). This was, however, not paralleled by an increased occurrence of higher hearing thresholds and the dosimetry recordings apparently did not reflect their average long-term day-by-day exposure to occupational noise. This may be because noise levels have increased in kinder gardens contradictory to the decreasing exposure levels that are expected in most industrial trades.

All noise-exposed workers were advised to wear hearing protectors permanently during the day of the audiometric testing. This may not have been sufficient to account for temporary threshold shift and we then overestimate the prevalence as well as the relative risk of permanent threshold shift (hearing handicap) by noise exposure.

Workers that reported they used hearing protectors during almost all years of employment in a noisy trade did not have lower risk of hearing handicap than other workers exposed at comparable levels and duration of noise. This indicates that available hearing protectors have not been properly used, at least in the past. The protective effect of hearing protectors is significantly reduced even by short periods off. [3],[10]

Our results were based on random sampling of companies within selected trades with expected high noise exposure levels followed by sampling of workers within the companies. Even if the study was restricted to workers and residents of Aarhus County, the results are expected to be representative and to provide valid estimates of the present burden of noise-induced hearing loss in the selected industries in Denmark. Noise-induced hearing loss may also be a problem in other industries and future extended surveys are needed to complete this first national Danish survey.

We classified workers by their trade mean noise exposure level because of earlier reports of a large day-to-day variability in noise exposure levels for individual workers. [17],[18] Additional analyses classifying the workers by their individual L Aeq -values (data not presented) showed weaker exposure-response relationships. This finding further supports our exposure classification strategy. Group level exposure assessment has been shown to yield unbiased relative risk estimates at the expense of inflated variance of risk estimates. [25] This is so because grouping leads to predominance of Berkson-type error in exposure assessment. [26]

All analyses were controlled for age, sex and reported ear disease and confounding by these factors can hardly explain our results, even if some residual confounding may still be present. Smoking has been suggested as a risk factor for hearing loss [27] or to modify the risk of noise exposure, [28] but inclusion of a smoking variable did not change our estimates considerably and was thus left out. [29] There was no indication that our results were confounded by noise exposure outside work, but this was not expected because the exposure levels were a magnitude below the occupational levels in accordance with the recent findings by Neitzel and Seixas. [30] We did not adjust for reported hearing protector use since this was a strong marker of noise exposure level and in this material did not modify the risk of hearing handicap.

Peak noise exposure is assumed to have a more deleterious effect on the sensoneural structures of the inner ear than continuous noise exposure. [31] We recorded peak values for every five seconds and constructed aggregated measures for the full work-shift of the workers. This measure was strongly correlated with the equivalent sound level and we were not able to segregate a sufficient number of workers exposed at comparable equivalent noise levels with and without peaks to assess the possible impact on the hearing threshold.

This study concludes that noise-induced hearing loss remains a prevalent problem in trades with well-known high noise exposure levels. Forty to fifty per cent of all cases of hearing handicap in these trades are probably caused by noise exposure at work. This study also suggests that preventive measures enforced during the past 10-15 years have born fruit since no increased risk of hearing handicap was neither seen in workers entering a noisy job during this time period nor in young workers. Ongoing surveillance of noise and hearing levels in appropriate populations should control if this holds true.


  Acknowledgement Top


The authors wish to thank audiologist Hanne Jessen and project secretary Jane Boilesen for company contacts, site visits and organisation of the dosimetry measurements and Freddy Wiesler for technical assistance with the planning of the noise measurements. The study was financed by a grant from the Danish Working Environment Authority (jnr 20000087619).

 
  References Top

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Correspondence Address:
Henrik A Kolstad
Department of Occupational Medicine, Århus Sygehus, Nørrebrogade 44, DK - 8000 Århus C
Denmark
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1463-1741.33538

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    Tables

  [Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5]

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14 Beroepsslechthorendheid bij zwemonderwijzers
Arnold Schriemer,Erik Kateman,Bas Sorgdrager
Tijdschrift voor bedrijfs- en verzekeringsgeneeskunde. 2010; 18(1): 16
[Pubmed] | [DOI]
15 Beroepsslechthorendheid bij zwemonderwijzers
Arnold Schriemer, Erik Kateman, Bas Sorgdrager
Tijdschrift voor bedrijfs- en verzekeringsgeneeskunde. 2010; 2010(1): 16
[VIEW] | [DOI]
16 Sorderas profesionales
J.-B. Nottet, A. Crambert, B. Lombard, C. Boursier, B. Suc
EMC - Otorrinolaringolog??a. 2010; 39(1): 1
[VIEW] | [DOI]
17 The epidemiologic study on hearing standard threshold shift using audiometric data and noise level among workers of Isfehan metal industry
Pourabdiyan, S., Ghotbi, M., Yousefi, H.A., Habibi, E., Zare, M.
Koomesh. 2009; 10(4): 253-259-+38
[Pubmed]
18 Noise-induced hearing loss: Are health service surveillance programs alwayseffective? | [Ipoacusia da rumore: I programmi di sorveglianza sanitaria sono sempreefficaci?]
Perbellini, L., Veronese, N., Raineri, E., Rava, M., Riolfi, A.
Medicina del Lavoro. 2009; 100(SUPPL.1): 20-23
[Pubmed]
19 Effects of shift work on noise-induced hearing loss
Chou, Y.-F., Lai, J.-S., Kuo, H.-W.
Noise and Health. 2009; 11(45): 185-188
[Pubmed]
20 Surdité professionnelle
J.-B. Nottet,A. Crambert,B. Lombard,C. Boursier,B. Suc
EMC - Oto-rhino-laryngologie. 2009; 4(3): 1
[Pubmed] | [DOI]
21 Sordità professionale
J.-B. Nottet,A. Crambert,B. Lombard,C. Boursier,B. Suc
EMC - Otorinolaringoiatria. 2009; 8(4): 1
[Pubmed] | [DOI]



 

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